I still think this is a disconnect supervision issue, maybe specifically on those last 2 trunks.

What version of voicemail are you using?

Another way to test your disconnect supervision is to abandon a call while on hold. Not exactly the same, but should tell you if you're getting the disconnect from the carrier.

I also suspect another option as there may be something off with your seconds of silence/non-silence and minimum message length settings. What are those set to?

Justin