Digitization of telephone networks is accompanied by a gradual migration from historical technologies such as RTC to more modern technologies (Voice over IP). The gradual shutdown of RTC, which is an obsolete technology, will promote the dissemination of more modern technologies: ultimately, theUsers will be free to choose the operator of their choice and the offer best suited to their situation (access to several services via a box: telephone, internet, etc., purely mobile use, etc.).

Voice over IP (VoIP or Voice over Internet Protocol) consists of transmitting a voice conversation over an IP protocol network (Internet Protocol) i.e. on a data network as opposed to transmission over the traditional telephone network or public switched telephone network (PSTN).
IP has the characteristic of transporting data in the form of packets.

The voice is therefore digitalized, compressed, sent in data packets over the network according to the IP protocol. The received data is decompressed and converted into audible voice.
All types of IP networks can therefore be used, private networks or public internet networks, as well as all types of network access, Frame Relay, ATM, wireless, fiber, etc.

The central element of traditional business telephony is the PABX.

There are three solutions for switching to VoIP:
1. Theadding an IP gateway to the PABX already present.
2. Replacing the classic PABX with an IP PBX (VoIP call server and application server) involving the replacement of traditional terminals with IP telephones.
3. Finally, theoutsourcing the functions of a classic telephone to an IP Centrex, a service provided by an operator or other VoIP solution provider, who manages the service end-to-end.