of GSM/VoIP networks allow the service providers to avoid the local Wired
Public Switched Network (PSTN) in communication with a GMS mobile handset. As a
result, the high rates charged by telephone companies are avoided.
A message which is played to the called party before answering the call, in the most cases the message tells who is calling, the name or telephone number helping you to decide how to answer to the call and sometimes whether to answer it at all.