Choppy MOH 1.4.2 (only from SIP trunk)

Hi, got this problem:
I have a time condition which sends to a queue with a MOH.
Now, if I dial the queue from an internal SIP phone, it sounds OK.
If I dial in from the VoIP trunk, MOH is choppy.

It is not bandwith/provider issue since I tested with an ol * machine (1.2) on the same LAN and no issues are there.

Any clue where to look ?