What happens during SIP REGISTER?

• A. An endpoint sends REGISTER to bind its contact address to its SIP identity
• B. It negotiates media codecs
• C. It opens a SIP dialog
• D. It subscribes to presence

Answer: (A)

Registering in SIP binds the endpoint’s contact address to its SIP identity, so the registrar knows where to reach that user. The device sends a REGISTER message to the domain’s registrar, providing its identity (the SIP URI) and one or more contact addresses in the Contact header, along with an expiration. The registrar stores this binding and uses it to route inbound requests for that SIP URI. The client typically refreshes the binding periodically (re-register) to keep it alive, which also helps with NAT traversal and mobility. Media codec negotiation happens during session setup, when a call is established via INVITE and the session description (SDP) is exchanged. Opening a SIP dialog occurs after the session is accepted, not during registration. Subscribing to presence is a separate mechanism (SUBSCRIBE/NOTIFY).