Okay... so heres me thinking my Asterisk box was ready for fulltime use, but not quite...

My x100p card (a Voxzone clone touted as being as close to the original as possible), and is in fact seen by Asterisk as a genuine card... is hooked up to TNZ PSTN.

Incoming or outgoing calls lose audio both ways (but doesnt drop the connection), after about 20 - 180 seconds on a call - even with both parties talking frequently.