SIP registrar

A SIP registrar is the bookkeeper of a phone system. Its job is to keep an up-to-date list of where every phone can be reached right now. When a Softphone or desk phone comes online, it tells the registrar its current network address, and the registrar writes that down so any call meant for that Extension knows where to send the ring.

This matters because the address of a phone can change. Agents work from different networks, addresses get reassigned, and laptops move between Wi-Fi and wired connections. Without a registrar keeping track, the server would have no reliable way to find a phone when a call comes in. The registrar is usually built into the same Asterisk server that handles the calls.

How a phone checks in

The check-in happens through Phone registration. The phone sends a Registration string that includes its username and password, and the registrar verifies them and stores the location. These registrations expire after a set time, so the phone re-registers regularly to stay reachable. If it stops checking in, the registrar eventually marks it as offline and calls stop reaching it.

Not every SIP (Session Initiation Protocol) connection works this way. A carrier link is often set up with IP authentication instead, where trust is based on a fixed, known address and no ongoing registration is needed. Use a registrar for devices that move, and treat each registered device as its own SIP peer on the server. Choosing the right one is part of how you pick and configure a connection, which we cover in our guide to choosing a SIP carrier.

When a phone will not ring even though the agent is logged in, the registrar is one of the first places to look. Either the phone never registered, or its registration expired and it failed to renew. The server keeps a list of registered phones you can inspect, and a phone that is missing from it cannot receive calls no matter how everything else is set. Common causes are a wrong password in the registration, a firewall blocking the renewal, or a network that keeps changing the phone's address faster than it can re-register. A phone that goes quiet quietly drops off the map until it checks in again.