Simple definition
A SIP registrar accepts endpoint registrations and maintains address-to-contact bindings.
Purpose and use
A SIP registrar maintains mappings between a SIP identity and contacts where the user can be reached.
How does it work?
It processes REGISTER requests and their lifetimes, making that information available to a location service.
Example and verification
For missing incoming calls, inspect the registered contact, expiry time and reachability from the telephony infrastructure.
Points to watch
A registrar differs from a proxy or media server even when one product combines these functions.
In short
A valid registration provides declared location information; it does not alone guarantee working signaling and audio routing.