Inbound ISDN call no audio

Hello Board,

i have a problem and i cannot get the solution by my own. I am using a xlite phone connected to my asterisk box. The asterisk box is connected to a sipgate account (prefix “8”) and to ISDN (prefix “9”) (euroISDN Fritz!Card USB).
I can call to anywhere via sipgate using prefix 8. Everything works fine. I can get calls from outside via spigate account, everything works fine. I can receive calls from ISDN on the ISDN MSN, sip phone is ringing, everything woks fine. Now there is the problem:

When i dial the prefix 9 and the phone number, the external ISDN phone is ringing at once. I take the call on the isdn phone. In the asterisk console i have to wait about 15 seconds until the connection is established and the xlite is now recognizing the connection as established. I can hear audio on the isdn phone but i can hear NO audio on the xlite. This happens only on incoming isdn calls. Outgoing isdn calls work normal.

I tried for days but i cant find a solution, perhaps anyone of you can help me fixing this problem. If this is done, the box is running.

Many Thanx