Unfortunately and especially unexpectedly bad

I've been around 20 years a very satisfied user of various Gigaset devices that were previously produced and sold, under Siemens Gigaset.
Currently I still use an old telephone system of the type "Siemens Gigaset 2060 ISDN". Initially with the then very comfortable handsets "Siemens Gigaset 2000C", which, however, unfortunately, in recent years were gradually broken. I replaced (broken on the board are probably interconnects) had to be replaced during the call by two handsets "Siemens Gigaset S44", but also, unfortunately, by temporary outages now. I opted for the purchase of two "Gigaset SL400 H handsets", amazingly, still allow for full functionality to use the old, above PBX. Numbers are displayed, the call list in the phone system is visible and also the ISDN features (call forwarding, call waiting, conference, etc.) are no problem! It will be displayed even the phone costs per running conversation and in total. I was very pleasantly surprised, because the compatibility database on the Gigaset website pointed out that the telephone system only partially communicate with the Gigaset SL400 H handset, and can be operated.
Since the Gigaset SL400 H handset also supports Bluetooth, I bought this also spontaneously ZX600 headset and was bitterly disappointed.
The couple worked fine, but the Sprachqualiät is to put it mildly a disaster. Permanent hisses and roars the connection as if you were standing in a heavy rain shower. The interlocutors say consistently that it was too quiet to hear. Also, even refers to the interlocutor very bad, even if you adjust the volume on the headset and the Bluetooth Configuration in the handset to maximum. The same applies to the setting of the microphone. Even if you set maximum sensitivity here so understands one of the caller on the telephone only rather quiet. You then switch the headset off and picks up the handset directly into their own hands, the voice and Hörqualiät is blatantly better, so as it should be. And for both sides of the connection. In addition, the manual is very poor, or content even wrong. It describes the function LED statuses that are not part as shown on the headset. On the other hand flashes or the LED lights up in blue or red in various intervals, in turn, are not described in the specification. So one wonders: What color and what interval mean so what function. This really solved totally evil. Could not have been here just can incorporate a few different colored LEDs, clearly representing the respective function? In addition, can the Invite Ecker (mini USB) that is difficult in fumble in the headset. You always have the fear, while breaking anything.
All in total very unsatisfactory. I sent then the headset back.
To me it's really a mystery how Gigaset was able to bring a bad device to market. That does not fit the usual quality of this brand. Urgently needs to be improved on the headset! So it is certainly not usable !!!
From buying so I can unfortunately only advise. I myself must now on the search for something to make better ...