Unrecoverable fault apparently at all MT-D

My new MT-D (operated on a FritzBox 7270) had the following error: After direct dial a number in the MT-D and pressing the call key will sound instead of a Feizeichens a loud, ugly, scratchy sound. The called party can accept the call however. But if I choose the same number (of another) on the call log, you will hear a normal dial tone and no loud croak / scratching. After a moment's utter ARGUS is switched on again and subsequent on the MT-D call key is the above. Error initially disappeared after a few hours but again available. A reset of the MT-D and then you log on to the base 7270 has brought nothing.
On the advice of the AVM-email support I have replaced the unit shortly after buying a new device. This was exactly the same mistake. On the advice of AVM I have the latest software replaced with a new beta firmware for the box and the Fon. The error RECOURSE the same.
AVM has proposed to return the unit for repair. That seems hopeless, because there already is a replacement unit. There is apparently a systematic error (the FritzFon is not compatible with the own Fritz box; other Siemens DECT function of the Fritz box the way properly), which can not be remedied by AVM. Thus, anyone who buys a MT-D for its FritzBox must assume that it arises a product with a built-in malfunction.