Yeah, this is pretty common actually. The first thing you need to do is, figure out the IP addresses involved in the media path. You didn't mention much about your infrastructure, but one way to do this is to pull your CallManager logs, and then load them into TranslatorX (translatorx.org), and then using the call list in TranslatorX, double click the inbound call record and check the media IP addresses of both sides. Then double check that there is routing between those two endpoints. Keep in mind though, that depending on a few factors, there can actually be more than 2 ip addresses inolved in the media path. Say for example, an MTP was invoked in the call flow, then there would be three ip addresses, however translatorX will only show two of the three, so you might have to play detective and figure out a little more about it.
You could also try loading your logs into
https://cway.cisco.com/csa/ for analysis, but it's not usually that good at finding problems. Worth a shot.
