|INVITE from TU                            
             Timer A fires     |INVITE sent                               
             Reset A,          V                      Timer B fires       
             INVITE sent +-----------+                or Transport Err.   
               +---------|           |---------------+inform TU           
               |         |  Calling  |               |                    
               +-------->|           |-------------->|                    
                         +-----------+ 2xx           |                    
                            |  |       2xx to TU     |                    
                            |  |1xx                  |                    
    300-699 +---------------+  |1xx to TU            |                    
   ACK sent |                  |                     |                    
resp. to TU |  1xx             V                     |                    
            |  1xx to TU  -----------+               |                    
            |  +---------|           |               |                    
            |  |         |Proceeding |-------------->|                    
            |  +-------->|           | 2xx           |                    
            |            +-----------+ 2xx to TU     |                    
            |       300-699    |                     |                    
            |       ACK sent,  |                     |                    
            |       resp. to TU|                     |                    
            |                  |                     |      NOTE:         
            |  300-699         V                     |                    
            |  ACK sent  +-----------+Transport Err. |  transitions       
            |  +---------|           |Inform TU      |  labeled with      
            |  |         | Completed |-------------->|  the event         
            |  +-------->|           |               |  over the action   
            |            +-----------+               |  to take           
            |              ^   |                     |                    
            |              |   | Timer D fires       |                    
            +--------------+   | -                   |                    
                               |                     |                    
                               V                     |                    
                         +-----------+               |                    
                         |           |               |                    
                         | Terminated|<--------------+                    
                         |           |                                    
                         +-----------+