SIP Registration Is a Device-Identity Checkpoint
A successful REGISTER proves more than network reachability because it joins device credentials to PBX state.
REGISTER
Tracing an IMS REGISTER Showed Me Where SIP Meets Diameter
An IMS registration capture connected familiar SIP REGISTER messages with the less visible Diameter exchanges used to select and authorize serving functions.
SIP Registration Expiry Explained Why Phones Disappeared After a While
A successful REGISTER is temporary state, so expiry, refresh timing and NAT mappings all matter if an endpoint is expected to remain reachable.