Siemens Gigaset SX 455 ISDN

Has anyone been similar incidents as I can observe:

It was a fax, which could no longer send. The cordless phones work on the understanding clean. The error is heard on the opposite side like a def. ISDN card.

After separating from the Gigaset ISDN was the error away. The understanding clean, the fax went through without problems.
Now in a short time the error twice in two different locations with different devices occurs, I suspect the error of the series and do not see it as an isolated case.

Who has the similar problem with Siemens Gigaset and can help me to eradicate the issue.

Exactly as the error for me, and also because of this description, type, I now even more on the Gigaset. the hardware is different, and always the same manufacturer is recommended, I think.

I had a few months since the Siemens Gigaset SX455 NTBA and additionally on an ISDN PBX (Telnet Max 8a / b) is connected. Since that time, I have trouble calls on the PBX. One is connected properly to understand but my voice comes totally distorted only when connect. Calls via the Gigaset are possible without any problems. Meanwhile, I had exchanged ISDN PBX against a Telnet Dax181. And the problem still exists. Has anyone similar problems with the Gigaset SX455 and perhaps a solution for me? Thank you in advance.