What is an ITSP?

ITSP stands for Internet Telephony Service Provider, the modern carrier that sells access to the phone network over SIP instead of copper. Here is what that relationship actually involves.

The carrier that actually reaches the PSTN

ITSP stands for Internet Telephony Service Provider. It is a carrier that sells access to the traditional phone network over SIP, so a business can send and receive real phone calls without ever touching a physical phone line. An ITSP is effectively the modern replacement for the phone company that used to run copper into your building.

What you are actually buying

An ITSP relationship usually comes down to a SIP trunk: a defined connection specifying how many simultaneous calls you can make, which numbers you own, and the exact SIP signaling details your equipment needs to match. Behind that trunk, the ITSP is responsible for actually getting your call onto the wider phone network, wherever it needs to go.

Why your SBC is usually the thing that peers with them

An ITSP does not trust your internal PBX directly, and your PBX was never built to defend itself against the open internet. Your SBC is almost always the device that actually registers with the ITSP and exchanges signaling, standing between your internal call control and their network.