Long story short: A new telephone was needed.
I did not need any frills as SMS writing (but what is theoretically possible), or retrieve emails (apparently the Festnetztelfone develop slowly to small smartphones). For that I have my phone. I wanted interference-free calls and it can connect via DECT.
I had read that the marks FritzFon and Gigaset are not as susceptible to interference of the type described above, and preferably with a FritzBox (Home Server) harmonize. Of course there were also the experiences reported similar problems. But I had no choice but to figure it out himself. I then decided to use the Gigaset A630a.
The delivery came as advertised on after 4 days. The first impression was very positive. The phones make a good impression processed. I have the base station directly connected to the router and the remote station via DECT. The decor was very easy. Two I have assigned its own number. The phone works properly. Finally, no more interference!
Conclusion: I had a positve experience, therefore a strong buy recommendation from me.
Another thing: If the phones are connected via DECT, the setting options are somewhat limited, but still fully sufficient. Because you'd have to buy a phone from FritzFon. This gives you full via DECT settings.