How it works
A SIP phone sends REGISTER to a registrar. Once authentication succeeds, the server associates the user’s SIP address with a reachable contact for a limited period. The endpoint must refresh the registration before it expires. Registration alone does not establish an audio session.
Business example
A remote employee opens a softphone at home. The corporate PBX can route incoming calls to the newly registered device.
What to consider
Review registration expiry, 401 authentication challenges followed by successful responses, NAT behaviour and registrar availability. A failed registration is not the same as an outbound dial-plan failure.