When a SIP phone starts, it first register itself with the SIP registrar.
1. SIP phone send HTTP request to SIP registrar
2. SIP registrar send SIP phone info to a location database (ip address etc)
3. ACK SIP phone
Basic call flow:
1. Invite from a SIP phone to SIP registrar (Proxy)
2. Proxy query location database and finds target SIP phone location
3. Proxy notify target SIP phone
4. Target SIP phone replies to Proxy, Proxy to inviting SIP phone
5. Inviting SIP phone ACK target SIP phone (connected)
Key Components:
1. SIP Phone (User agent)
2. SIP Server (registrar, proxy)
3. PSTN gateway
4. App servers (media server, etc)