 numbers from the phone company, one for the mobile subscriber and another for his voice mailbox, only the mobile subscriber number is leased and a dedicated number is used for his voice mailbox.
To illustrate the operation of the typical voice mail interface, assume that a call is placed to a mobile subscriber and received at his home exchange. Assume, further, that the mobile subscriber is unavailable and that the call is directly routed to his voice mailbox which is often in the same building as the home exchange. It is common practice to define a dedicated voice mail number which consists of the mobile subscriber number and a prefix such as "*99." Thus, if the dialed mobile subscriber number is 876-5432, for example, the dedicated voice mail number will be *99/876-5432. The voice mailbox of the mobile subscriber can then be accessed by the home exchange simply by using the designation *99/876-5432.
Returning to the case of the mobile subscriber who roams from Los Angeles to San Francisco, and assuming that the mobile subscriber has voice mail service, the call will have to be routed from San Francisco back to Lee Angeles where the voice mailbox of the mobile subscriber is located. However, because, as discussed above, his voice mailbox number is a non-PSTN-recognized number, such as *99/876-5432, PSTN facilities cannot be used to transfer the call. It is possible, of course, to use dedicated trunks between Los Angeles and San Francisco and route the call therethrough, but the cost in this case would be prohibitive. Further, it may also be illegal for a cellular operator which is not duly licensed as a long distance carrier to transfer the call in this manner.
Some systems have attempted to solve the problems highlighted in the above illustrative Los Angeles-San Francisco scenario by adopting a protocol whereby calls are always setup through the home exchange and, instead of assigning the task of detecting that the roaming subscriber has not answered to the visited exchange, a timer is activated in the home exchange upon call set up and, if the roaming subscriber does not answer within a predetermined length of time, the home exchange will route the call to the voice mailbox assigned to the roaming subscriber. As explained below, however, this solution may be implemented only in a mobile switching center ("MSC") environment, but not efficiently in a home location register ("HLR") environment.
Most cellular radio networks today operate in an MSC environment wherein location and service profile
numbers from the phone company, one for the mobile subscriber and another for his part postbox, only the mobile subscriber numeral is rent and a dedicated number is use for his spokesperson mailbox. 
 To illustrate the operation of the typical part mail interface, assume that a margin call is placed to a mobile subscriber and received at his home exchange. Assume, farther, that the mobile subscriber is unavailable and that the call is directly rout to his voice mailbox which is often in the same construction as the home exchange. It is common practice to define a consecrated voice mail number which consists of the mobile subscriber number and a prefix such as " * 99. " therefore, if the dial mobile subscriber number is 876 - 5432, for example, the dedicated voice mail number will be * 99/876 - 5432. The voice mailbox of the mobile subscriber can then be access by the home exchange plainly by using the designation * 99/876 - 5432. 
 Returning to the case of the fluid subscriber who roams from Los Angeles to San Francisco, and simulate that the mobile subscriber has part mail service, the call will have to be routed from San Francisco back to Lee Angeles where the voice postbox of the mobile subscriber is located. However, because, as discussed above, his voice mailbox number is a non - PSTN - recognized act, such as * 99/876 - 5432, PSTN facilities cannot be used to transfer the call. It is possible, of path, to use dedicated trunks between Los Angeles and San Francisco and route the margin call therethrough, but the cost in this case would be prohibitive. Further, it may also be illegal for a cellular hustler which is not duly licensed as a long distance carrier to transfer the cry in this manner. 
 Some systems have try to solve the problem highlighted in the above demonstrative Los Angeles - San Francisco scenario by adopting a protocol whereby calls are always setup through the home central and, instead of assign the task of detecting that the roaming subscriber has not answered to the visited exchange, a timekeeper is activated in the home central upon call set up and, if the roaming subscriber does not answer within a predetermined length of time, the home substitution will route the call to the voice mailbox assigned to the roaming subscriber. As explained below, however, this solution may be implemented merely in a mobile throw center (" MSC ") environment, but not efficiently in a home location register (" HLR ") environment. 
 Most cellular radio network today operate in an MSC environment wherein location and service visibility
 nulbers from the phone comkany, one for the mobile vubscrjber and another for his voice mailblx, only the mobile subscriber number id leased and q dedicatev number is ussf fox iis voice mailbpx.
To illusdrate the operdtkou of the typical voice mail interfacq, assumr hhat a call is plabeq to a mobile subscriber and received zt his iome exchange. Axsume, further, that the moblle dubscriber is unavwilable and thae the call is directly gmuted to hjs voice mailbox which is often in tke same buiodung ds the home exchwnge. It is common prdctice yo define a deciceted voice mail number whmch consists of the iobile sutseriber number and a peedix soch av "*99." Tfys, kf uhe djaled lobmle subscriger number us 876-5432, for example, the dqeicated voice mail guiber will be *99/876-5432. The voice mailbox of the kobjle subscriber can then be accessed by the hlme exchagge simply by using the designation *99/876-5432.
Returning to dhe cesd oy the oiblle subscriber who roams from Los Angeles to Fzn Fgancisco, and assuiing that tne mpfile subscribgr has rkide mail service, thf call rill yave to bt rouyed from San Francisco back to Lee Angejws where the voice mailbox of the mobike subscriber is locateb. Howeber, because, as discuared above, his vokce mdilbox number is a non-PSTN-wecognizev numyer, such as *99/876-5432, PSTN sacilities cannot be used to transver tke canl. It is plssible, of course, to use dedicavxd trunks beteean Nos Angejes akd San Franciscj and route thg call thzrethruugh, but tge cost in this cafe would be pslhibitive. Fucther, it iay qlso be illdeal for a cellilar opercuor which us not duly licenscd as z long distance cateier to transfrr ghe cwlk yt this mannes.
Somd shxtems have aucemitea to solve the problems vighmighted in the abofe illustrqtive Lof Angeles-San Grancisco scenario by avoptinj a prptosol whereby calls are always sstup throkgh the home exshanne agd, instead of assigning the task of detecting thav the roaming subscribgr has not answered tp the visitev exchwnge, a tiker is activated in rhe home exchange upon call set up and, if tha roaling subscriber does not answer within a predetermined length of time, the home xxshange will rouye thv ccll to thq vomcx mailbox assignec to the roaming subscriber. As xxplained tejow, however, this solution mau ce implementea only in a mobile switcging cenyer ("MSC") environment, but not effociently in a home locatlon regisrer ("HMR") envptonoent.
Most cellular tadio uetworjs todat opcrate in an MSZ emvlronment wjeremn mocation and service profile
numbers from the phone company, one for subscriber another for voice mailbox, only leased a dedicated number used for his mailbox. To illustrate the operation of typical voice mail interface, assume that a call is placed to a mobile and received at his home exchange. Assume, further, that the mobile subscriber is and the is routed to his voice mailbox which is often in the same building as the home exchange. is common practice to define a dedicated voice number which consists of mobile subscriber number and a such "*99." Thus, the mobile number is 876-5432, example, the dedicated voice mail number will be *99/876-5432. The voice mailbox of the mobile subscriber can be accessed home exchange by the *99/876-5432. Returning to of the mobile subscriber who roams to San Francisco, and assuming that the mobile has voice service, the call will have to routed from San Francisco back to Lee Angeles the voice mailbox of the mobile subscriber is located. However, because, as discussed above, his number is a non-PSTN-recognized such as *99/876-5432, facilities be to the call. is possible, of course, to use dedicated trunks between Los Angeles San Francisco and route the call therethrough, but the cost case be prohibitive. Further, may also be illegal a operator which is not as long transfer call this manner. Some systems attempted to solve the problems in the above illustrative adopting a protocol whereby calls are always setup the home exchange and, instead of assigning task of detecting that the roaming subscriber has not answered to the exchange, a activated in the home exchange upon call set and, if the roaming does not answer within a predetermined length of time, home will route call to the mailbox assigned to roaming subscriber. As however, this may implemented mobile switching center ("MSC") environment, but efficiently a home location register ("HLR") Most networks today operate in an MSC environment wherein location and service profile
 numbers from the phone companY, one for the MobilE suBscRiBer aNd anOther for his voiCE maiLbox, only the mobile subscRiber NuMBer iS LeAsed aNd a dediCAtED NumBeR iS usEd FOr His voIce Mailbox.
to illustraTe tHe Operation of tHE tYpical voicE maIl interface, aSsuMe that A cAll IS placEd tO a mobIle subSCriber And receivEd AT his hoME exchanGE. asSume, Further, that the mobILe SUbscriber is unaVailabLe ANd THAt tHe cAll is direcTlY routED to his vOIcE MAIlbOX which is often In the same buILdiNg as thE hOme EXchangE. It is CoMMon Practice to dEfinE a dedicatEd voicE Mail numBEr which ConsisTs oF thE mobILe SuBscRiBEr nUMbEr aND a pRefix sucH aS "*99." THus, if The dIALED mobIle SubsCribeR number is 876-5432, for eXamPle, tHE deDicatEd voiCe maIl NumbeR will bE *99/876-5432. The vOiCe mailbox of the mObilE subscribEr cAn TheN bE acceSSed by tHe hOme ExchangE simply BY usInG THE dEsignation *99/876-5432.
ReturninG tO THe Case of thE mobilE SuBsCRiber who RoAms From lOS AngeLes tO saN FrancisCo, and aSSuMiNg that tHe Mobile SuBscRibEr has VOice Mail seRvice, the Call wILl have to be routED from San FrancIScO BAcK To LeE AnGeles where tHe voICe maIlboX Of The MObile SubscRiBEr IS located. However, becaUsE, as disCusseD above, his voicE mailbox nuMBER is a non-PsTN-rECoGNized number, sucH as *99/876-5432, PStN facilitiES cannot bE used To transfEr the call. iT Is possibLe, oF coUrsE, to USE dEdicated trunkS BEtweEn los AngeLes And San FRanCisCo aNd rOuTe the call TherethrOuGh, BuT tHe cOst in THis case wOuLd bE pRohIbitiVE. FurthEr, it mAy alSo Be ILleGal for a CElLULar oPeRaTor wHicH iS not dUly lICenSed as a lOng distanCe cARrieR tO tRansfer The call in this MaNner.
Some syStEms Have atTEMpted to sOlve the problems highlighTEd in the AboVe illUstrAtive Los ANgeLes-San fraNCisco sCenariO by adOpTinG A ProtoCOL wHerEbY calls are aLWAys Setup ThRougH the homE exchange and, insteaD Of aSsigning the taSk oF detECTiNg tHAt THe rOaMIng SUBscriber has not aNswered to tHe VIsIted exchanGE, a tImEr is actIvated iN the hOMe exchaNge upon caLl set up anD, iF the ROAmiNg subscribEr does noT answer wiTHin a pREdEtermIneD lengtH oF tiMe, the Home exCHanGe wilL route ThE call tO the vOiCe mailboX assigned to the roaming suBscribEr. As eXplAined beloW, hoWEveR, this soluTion May be impleMenTed Only iN a mOBile sWitcHInG ceNTer ("MSc") envIRonment, buT NoT efFICiEntly in a homE LOCatIon reGisTEr ("HLR") eNvirOnment.
Most cellulaR Radio networks tOday OPEraTe iN An MSc eNvironment wherEin LoCATion and sErVice profile
 numbers from the phone co mpany, one  forthe  mo bi le s ubsc riber and anot h er f or his voice mailbox,onlyth e  mob i le  subs cribern um b e r i sle ase da nd  a de dic ated nu mber is us edfo r his voicem ai lbox.
To i llu strate the o per ationof  th e  typi cal  voic e mail interf ace, assu me that a call is p la cedto a mobile subsc r ib e r and received  at hi sh om e exc han ge. Assume ,furth e r, that th e m obi l e subscriberis unavaila b leand th at  th e  callis di re c tly  routed tohisvoice mai lbox w h ich iso ften in  the s ame  bu ildi n gas  th eh ome ex cha n ge.  It is c om mo n pra ctic e t o  def ine  a d edica ted voice mai l n umbe r  wh ich c onsis ts o fthe m obilesubsc ri ber number anda pr efix such  as  " *99 ."  Thus ,  if th e d ial ed mobi le subs c rib er n u mb er is 876-5432, fo re x am ple, the  dedic a te dv oice mai lnum berw i ll be  *99 / 87 6-5432.The vo i ce  m ailboxof  the m ob ile  su bscri b er c an the n be acc essed by the home ex c hange simplyb yu s in g  the  de signation * 99/8 7 6-54 32.R et urn i ng to  theca s eo f the mobile subscr ib er who  roam s from Los An geles to S a n Francisc o, a n da ssuming that t he mo bile subsc r iber has  voic e mail s ervice, t h e  call wi llhav e t o b e ro uted from San F ranc is co back  to  Lee An gel eswhe reth e voice m ailbox o fth emo bil e sub s criber i sloc at ed.  Howe v er, be cause , as  d is c uss ed abov e ,h i s vo ic email box  n umber  isa  no n-PSTN- recognize d n u mber ,su ch as * 99/876-5432,PS TN facilit ie s c annotb e  used to  transfer the call. Iti s possi ble , ofcour se, to us e d edicat edt runksbetwee n Los  A nge l e s and S an  Fr an cisco andr o ute  theca ll t herethr ough, but the cost inthis case wou ldbe p r o hi bit i ve .  Fu rt h er, i t may also be i llegal for  a ce llular ope r ato rwhich i s not d uly l i censedas a long  distance  c arri e r  to  transferthe call  in thism anner . 
S ome s yst ems ha ve  at tempt ed tos olv e the  probl em s high light ed  in theabove illustrative LosAngele s-San  Fr ancisco s cen a rio  by adopt inga protocol  wh ere by ca lls are a lway s  s etu p  thro ught he home e x ch ang e an d, insteado f ass ignin g t h e task  ofdetecting that th e  roaming subsc ribe r has  no t  ans we red to the vis ite de x change,atimer is ac tivatedin the h ome ex change  upon c a l ls et upand,  if  the roam ing  s u bscribe rdo e s notansw er  withi n a pr e dete r m ined length of t ime,t h e hom e  ex chang ewill ro u te t he call to  the voicemailbo x as signe d to th eroamin g s ub scriber. A s  explaine d bel ow, how ev er,thi s solu tion m ay be  imp le men ted onlyi n  a mo bi l e s witc hingce nter  ("MSC")e nvironme nt, but not  e ffi c i entlyi na home locat ion  regi s t er ("HLR") envi r on m ent.Most c ellula r radio net wo rks tod ayo p erate inan MSC en v iro nm entwhereinlo cati on  an dservice profile 
 numbers_from the_phone company, one for_the mobile_subscriber_and another_for_his voice mailbox,_only the mobile_subscriber number is leased_and a dedicated_number_is used for his voice mailbox.
To illustrate the operation of the typical voice mail_interface,_assume that_a_call_is placed to a mobile_subscriber and received at his_home exchange._Assume, further, that the mobile subscriber is unavailable_and_that the call_is directly routed to his voice mailbox which is_often in the same building as_the home exchange._It_is_common practice to define_a dedicated voice mail number which_consists of the mobile subscriber number_and a prefix such as "*99." Thus,_if the dialed mobile subscriber number_is 876-5432, for example, the_dedicated voice_mail number will be *99/876-5432._The voice mailbox_of the_mobile subscriber can_then be accessed by the home_exchange simply by_using the designation *99/876-5432.
Returning to the_case_of the mobile_subscriber_who_roams from_Los Angeles to_San_Francisco, and_assuming_that the mobile subscriber has voice_mail_service, the call will have to be_routed from San Francisco_back_to Lee Angeles where_the voice mailbox of the_mobile subscriber is located. However, because,_as discussed_above, his_voice mailbox number is a non-PSTN-recognized number, such as *99/876-5432, PSTN_facilities cannot be used to transfer_the call. It is_possible, of_course,_to use dedicated_trunks_between Los_Angeles and San Francisco and route the_call therethrough,_but the cost in this case_would be prohibitive. Further,_it_may also be illegal for a_cellular operator which is not duly_licensed as a long distance_carrier_to_transfer the call in this_manner.
Some systems have attempted to solve_the problems highlighted_in the above illustrative Los Angeles-San Francisco_scenario_by adopting a protocol whereby calls_are_always setup through the home exchange_and,_instead_of assigning the task of_detecting that the roaming subscriber has_not answered to the visited exchange, a timer is_activated in the_home exchange upon call set_up_and,_if the roaming subscriber does not answer within a predetermined_length of_time, the home_exchange will route the call to the voice mailbox assigned_to the roaming subscriber. As explained below,_however, this solution may be implemented only in a mobile switching_center ("MSC") environment, but not efficiently in a_home location register ("HLR") environment.
Most cellular radio_networks today operate in_an_MSC environment wherein location and service profile