distorted audio in one direction

Hello,

I'm getting one-way audio with the following setup:

german Telekom ISDN <-> DrayTek Corp miniVigor 128 ISDN <-> asterisk
with current mISDN and lcr versions.

SIP Calls with a softphone via asterisk to any ISDN Telephone work and I 
can hear the audio coming from the ISDN telephone. The audio send to the 
ISDN Telephone is distorted noise.

Any idea how to track this problem?

Andreas

andi@grieg:~$ uname -a
Linux grieg 2.6.39-bpo.2-kirkwood #1 Thu Sep 15 18:28:02 CEST 2011 
armv5tel GNU/Linux

andi@grieg:~$ lsusb
Bus 001 Device 010: ID 0675:1688 DrayTek Corp. miniVigor 128 ISDN TA

andi@grieg:~$ sudo lcr query

** LCR  Version 1.12

-> Using 'misdn_info'

Found 1 port
   Port  0 'HFC-S_USB.1':     TE/NT-mode BRI S/T (for phone lines & phones)
                               2 B-channels: 1-2
                                 B-protocols: RAW HDLC X75slp L2:DSP 
L2:DSPHDLC

andi@grieg:~$ sudo asterisk -V
Asterisk 1.8.15.0