atlanta.com  . . . biloxi.com
           .      proxy              proxy     .
         .                                       .
 Alice's  . . . . . . . . . . . . . . . . . . . .  Bob's
softphone                                        SIP Phone
   |                |                |                |
   |    INVITE F1   |                |                |
   |--------------->|    INVITE F2   |                |
   |  100 Trying F3 |--------------->|    INVITE F4   |
   |<---------------|  100 Trying F5 |--------------->|
   |                |<-------------- | 180 Ringing F6 |
   |                | 180 Ringing F7 |<---------------|
   | 180 Ringing F8 |<---------------|     200 OK F9  |
   |<---------------|    200 OK F10  |<---------------|
   |    200 OK F11  |<---------------|                |
   |<---------------|                |                |
   |                       ACK F12                    |
   |------------------------------------------------->|
   |                   Media Session                  |
   |<================================================>|
   |                       BYE F13                    |
   |<-------------------------------------------------|
   |                     200 OK F14                   |
   |------------------------------------------------->|
   |                                                  |