Registrar
A SIP registrar accepts REGISTER requests and stores where each user can currently be reached, so incoming calls can be routed to the right device.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
A registrar is the SIP server that accepts REGISTER requests and stores the binding between a user's address of record and the contact address where that user can currently be reached.
The registrar is the part of a SIP server that keeps track of where users are. When a softphone starts, it sends a REGISTER request containing its address of record, such as sip:201@pbx.example.com, and a Contact header with the IP address, port and transport it is listening on. The registrar checks the credentials, stores the binding, and returns 200 OK with the expiry time it accepted.
RFC 3261 section 10 describes this process. Bindings are temporary. The client must refresh them before they expire, typically every few minutes to an hour, or the registrar forgets the device and incoming calls can no longer be delivered to it. A user may have several bindings at once, one per device.
In most deployments the registrar is built into the PBX or the provider's SIP platform rather than being a separate machine. When a softphone shows a green registered state, it means the registrar accepted the most recent REGISTER. A 401 or 403 response at this stage usually points to wrong credentials or a server rule that rejected the request.
Sources and further reading
- RFC 3261: SIP: Session Initiation Protocol, Section 10 Registrationswww.rfc-editor.org
This article is published by SpaGreen Creative, the maker of SIPdia. Product names mentioned belong to their owners. Where it describes other software, treat the steps as general guidance and check the official documentation linked above.
