SIP trunk

A SIP trunk is the logical link that connects a company's phone system (IPBX or software switchboard) to the carrier's network, over the internet, using the SIP protocol. It replaces the physical trunks of the past — PSTN (analog) or ISDN (T0/T2) lines — with purely software channels. You no longer rent a cable: you subscribe to a number of simultaneous channels you can adjust in minutes — on a call-peak day or a busy season, you add some; the next day, you remove them.

Two consequences deserve attention. The first is geographic: a SIP trunk isn't tied to an address, numbers survive an office move, and handsets work anywhere there's connectivity. The second is security: once the trunk is open to the internet, it becomes a target — toll fraud on SIP trunks (international calls billed to the line's owner) is a classic risk, contained by destination restrictions and call caps. With the PSTN shutdown in France, the question is no longer whether to migrate, but which carrier to pick — one that secures the trunk and guarantees quality of service.