asterisk users - Feb 2004 - Voicemail cutting off messages on SIP

We have a situation where voicemail coming in (i.e. 
FXO->Asterisk->Voicemail) through a Mediacodes MP108-FXO are getting cut 
off a couple of seconds early. I recall a thread about this quite a while 
back where this was happening due to silence detection on ZAP channels... 
Has anyone experienced this and/or found a solution?

The MP-108 is using Polarity reversal, but no silence detection. Also, this 
problem doesn't happen on internal messages (from SNOM 200 phones).

Thanks,
--Ernest