Strange Sound level problem

Hi,

I got a very strange problem :
If I receive a call from isdn trunk , the caller can hear me quite fine.
If I make an outgoing call, using the same channel and to the same number, the callee can’t hear me because the sound level is very low for him.
On my side, the sound level is identical for incoming an outgoing calls.
It’s seems this problem more obvious on cellular phones.
I use AAH 2.8 bristuffed, and a Junghanns quadbri card.

Thanks for help