1. The legacy of the telephone network

The switched telephone network (or PSTN) and its mobile counterpart (GSM) have made telephony and its associated services universally accessible. The telephone call is the basic historical service. Other services, such as call forwarding, call transfer, call conferencing, prepaid services and toll-free services, have subsequently enriched the possibilities offered by the telephone network. These services are a significant source of revenue for operators, and have become a way of life for users, especially in the workplace.

Telephony over IP and voice over IP solutions could only be credible if the new techniques proposed provided at least identical functionalities. Standardization efforts, led by the ITU-T and the IETF in the 2000s, focused on defining protocols that could provide these same services over IP, and, in some cases, interoperate with existing PSTN services....