Answer supervision is not being provided by the Public Switched Telephone Network (PSTN) switch or PBX.
The router is expecting some kind of M lead response after it has placed the call toward the PBX. If it does not get a change of state, it assumes the call failed or was never picked up (no answer signal came back) and it drops the call.
To resolve this issue, choose from one of these options:
Configure the PBX so it provides this indication on the tie trunk when the called party extension answers.
Load a Cisco IOS Software version later than 12.07XK (12.1T or later on the 17xx) and issue the auto-cut-through command to tell the router to ignore the M lead response.