 on busy, where calls are diverted to the specified number speech coder tailored to voice characteristics. They therefore pro- when the mobile is already engaged on a call. As an alternative, vide a fully acceptable telephony service, but non-voice signals can networks also provide call waiting. suffer distortion across the non-audio transparent path. 5. Call waiting, where if a call is received when the mobile is already engaged on a call, the user is informed that a second call 47.6.1 Supplementary services is waiting, and can choose to place the first call on hold whilst dealing with the second caller. 6. Three party calling, where the mobile user may set up calls to Supplementary services are provided by means of enhancements to two other parties and connect them in a three way conference. the basic call processing software in the MSCs. Many of these This service can also be used to make enquiry calls whilst services have specific relevance to the cellular radio user, and in the holding the original call. main they parallel services which are becoming increasingly avail- able on the fixed telephone networks (such as BT's Star Services in the UK). 47.6.2 Value added services Typical services are as follows: Value added services are normally provided by means of peripheral 1. Call divert, where all calls are diverted to the specified number, units attached to the cellular network, or to the fixed network with which may be another mobile or a termination on another which to cellular network interconnects. In some countries, the network. This is of use if the user wishes to make calls but not prevailing regulatory regime will influence what services may be receive them. offered and in what manner, however, the following are typical. 2. Divert on no answer, where calls are diverted to the specified number when the user does not answer within (for instance) 20 seconds. This is of use if a mobile is left switched on in an 47.6.2.1 Messaging services unattended vehicle. 3. Divert on mobile unavailable, where calls ar