Disappointed by the phone Fritzfon MT-F

I have a modem router Fritz! Box 7360 and I am delighted that I had four connected cordless telephones (DECT) and Panasonic cordless telephone (DECT) Siemens Gigaset. Until then, everything worked.
To take advantage of all the possibilities of the router, I then bought a Fritz! Fon MT-F (the same brand AVM).

This purchase is extremely disappointing:
1. The sound quality is very bad. My correspondents often complain when all goes well if I use the Siemens or Panasonic. However, that the AVM Fritz! MT-F can even give her an HD sound quality ...
2. The quality of plastic is poor. The battery cover is already broken.
3. The electrical contacts of the charging station are not francs. It is often necessary to reset the phone, and above that the phone battery starts charging.

I have no problems with these three phones without Siemens and Panasonic wire that are also connected to the base Fritz! Box 7360 and who are much older.