After many problems with the Fritz! Box 7390 and VOIP (which I use exclusively), I made my decision to use a TP-Link WDR3600 N600 router which has no telephony on board, but it sends a strong wireless signal. As a replacement for telephony I angeschaft me this Cisco SPA112. Since I am a customer with Sipgate, I could use the instructions provided there to set the SPA112. Raustelefonieren worked immediately. Quality 1a.
But no ringtone for incoming calls. Here is the solution for it. Man goes into the web interface of the SPA112, there under VOICE - REGIONAL. There you change two settings. Under the heading "Ring and Call Waiting Tone Spec" further down the page you will change the sound type of "trapezoid" in "sinusoid". The "Ring Voltage" I downloaded "60" is set to. The "Ring Frequency" up to "25". The adapter is set innately to US standards, and that has (T-Sinus 45) guided at least in my case, where I use a very ancient but DECT phone to the fact that just came through no ringtone.
To activate on the second terminal of the SPA112 a fax machine, you go back to the web interface of the SPA112, there under VOICE - LINE2 (if that's Fax as attached). There you scroll down until almost the end of the form, to the heading "Audio Configuration". In the right column there is the setting "Enable T38 fax". Here to "yes" change.
Also still adjust all the other parameters for the second line according to instructions with Sipgate. And I have set for the fax machine to port 5161 (instead of 5160, which was meant for the phone with sipgate).
According to the amendments phone and fax function with me. Hope someone can help.