I had a problem with my IP telephony system the other week. For some reason, calls were working fine around all my sites except between two particular offices.
The first question I asked was: "What's changed"? There was one obvious answer - the WAN connectivity in one of the affected offices had been radically modified. However, the service provider checked and told me that all was in order and all the routes were correct. So we checked a whole bunch of other stuff, to no avail; unfortunately this particular brand of phone system has pretty rudimentary diagnostics (it can "ping" but it can't "traceroute", for instance) so troubleshooting was a pain in the butt.
In the end I'd exhausted all the other alternatives, and I asked this particular service provider to check again. After a more thorough search the second time around, a missing route was found. In the bit of the network that had been changed. Bah.