1.2.4.11:31361                 1.2.3.15:5060            1.2.5.156:5369
|        INVITE (SDP)         |                             |
| --------------------------> |                             |
|      401 Unauthorized       |                             |
| <-------------------------- |                             |
|             ACK             |                             |
| --------------------------> |                             |
|        INVITE (SDP)         |                             |
| --------------------------> |                             |
|         100 Trying          |                             |
| <-------------------------- |                             |
|  183 Session Progress (SDP) |                             |
| <-------------------------- |                             |
|                             |        INVITE (SDP)         |
|                             | --------------------------> |
|                             |         100 Trying          |
|                             | <-------------------------- |
|                             |         180 Ringing         |
|                             | <-------------------------- |
|  183 Session Progress (SDP) |                             |
| <-------------------------- |                             |
|                             |        200 OK (SDP)         |
|                             | <-------------------------- |
|                             |             ACK             |
|                             | --------------------------> |
|        200 OK (SDP)         |                             |
| <-------------------------- |                             |
|             ACK             |                             |
| --------------------------> |                             |
|           INVITE T.38       |                             |
| --------------------------> |                             |
|                             |           INVITE T.38       |
|                             | --------------------------> |
|                             |   488 Not Acceptable Here   |
|                             | <-------------------------- |
|                             |             ACK             |
|                             | --------------------------> |
|   488 Not Acceptable Here   |                             |
| <-------------------------- |                             |
|             ACK             |                             |
| --------------------------> |                             |
|        INVITE (SDP)         |                             |
| --------------------------> |                             |
|        200 OK (SDP)         |                             |
| <-------------------------- |                             |
|             ACK             |                             |
| --------------------------> |                             |
|             BYE             |                             |
| --------------------------> |                             |
|           200 OK            |                             |
| <-------------------------- |                             |
|                             |             BYE             |
|                             | --------------------------> |
|                             |  481 Call Leg/Transaction D |
|                             | <-------------------------- |