 have also been seized by the central office to send an incoming call to the PBX. If so, the PBX knows not to use the trunk for the outgoing call. It is also possible to use loop start trunks in a PBX. Loop start trunks are more typically used on a Key System for a smaller organization. When you press down on the outside line button on the telephone in a Key System, the outside dial tone is there. You do not need to dial 9 and the telephone system is not signaling to the central office, as with a ground start trunk. If you use loop start trunks with a PBX, there is a risk that someone calling in on 208 The Telecom Handbook Figure 10.1 Combination Trunks Combination Trunks Used for EXT. 241 Both Incoming and Outgoing Calls Dial "9" to Select a Combination Trunk to Place an Outgoing Call <<<<< Local Telephone Co. Central Office Incoming Call Goes to Switchboard. Switchboard Attendant Then Sends Caller to Requested Telephone Extension. OUTSIDE LINES a trunk will collide with someone who has just dialed 9, select- ing the same trunk to place an outgoing call. The callers will be able to speak to each other, although that it not what they in- tended! This type of collision is called glare. It will not happen on a ground start trunk. Most groups of combination trunks have a separate seven- digit telephone number associated with each trunk (seven digits plus 3 digit area code). If the central office runs out of telephone numbers, it may provide coded trunks. These trunks have no as- sociated telephone number and are reached only by dialing the main number and then rolling over if the main number is busy. A combination trunk is a switched circuit. It is one type of dial tone line. Direct Inward Dial (DID) Trunks Direct inward dial trunks are used for incoming calls only. Although in some parts of the United States (like New York City) they cost up to three times as much as combination trunks, they do provide a special function. Using direct inward dial trunks enables everyone within a company to have his own separate sev