Voipbuster - Again - audio issue

I hope someone can help me out, not looking for a solution (would be nice)
just a bit to understand more.
I am using a PA1688 based SIP device

Have it set up for what i think are all the settings for VOIPbusters.com
I can make call, they can hear me no problem
I can hear the message this call is 0 cents/minute
But then nothing, the call is still active until I hang up

The same thing happens with 18866.com but this did used to work until they
ditched being proactive with being an open SIP system
Tried using Stun,
Tried with different codec G729 G711a/u without success

I am guessing the server making the billing charges may not be the same as
that used for the actual call and that the line is transfered

any help appreciated