SIP Verbindungsaufbau

A (Caller)                          SIP Proxy                        B (Callee)
   |                                    |                                 |
   | 1. INVITE (SDP Offer)              |                                 |
   |----------------------------------->|                                 |
   |                                    | 2. INVITE (SDP Offer)           |
   |                                    |-------------------------------->|
   |                                    |      3. Trying (100)            |
   |<-----------------------------------|                                 |
   |                                    |      4. Ringing (180)           |
   |<-----------------------------------|                                 |
   |                                    |  5. OK (200, SDP Answer)        |
   |<-----------------------------------|                                 |
   | 6. ACK                             |                                 |
   |----------------------------------->|                                 |
   |                                    |  7. ACK                         |
   |                                    |-------------------------------->|
   |                                    |                                 |