Whilst it's becoming ever less common, sometimes there's a need to integrate with legacy equipment, such as PBXs & voice mail systems via a QSIG trunk. Given that a QSIG trunk requires an ISDN interface on a router with PVDMs, it's logical to stick an extra interface into your PSTN gateway & cable it to the legacy device.
Problem is that with MGCP you've now got 2 endpoints with the same IP address, which causes problems with the audio streams. If you enable debug isdn q931 you'll see calls come in a voice port & exit the other, yet there's no audio between the callers. Fortunately the solution is easy - in the CallManager service parameters enable path replacement & tweak the minimum/maximum delay timers: