Strange problem with MOH

Hello, I’m using FreePBX via the Schmooze distribution and I’m facing a very strange problem.
Some of my users are reporting that, after some time on the call they start hearing the MOH instead of the person on the other side and, the person on the other side says it all goes mute.
The impression it gives is as if the other person (external to our infrastructure and FreePBX) has somehow pressed OUR MOH button on the telefone, and placed our user on MOH!
This is driving me crazy, as I haven’t a clue on what can be happening, could you guys help me out find what may be going on?


Just found out this on the asterisk logs:
[2013-01-31 09:30:15] VERBOSE[6393] res_musiconhold.c: – Started music on hold, class ‘default’, on SIP/0223-0001924e
[2013-01-31 09:30:16] VERBOSE[6793] res_musiconhold.c: – Stopped music on hold on DGV/7

So it really starts the MOH, but why?