Since there was no other answer, let's say that the most easy solution is to simply avoid using the Fritzbox as a VoIP gateway because the result will depend on the modem/router used.
If you have to connect your landline phone via VoIP, use a Cisco SPA112, it's more reliable.
By the way, I have now returned my D7800 and am using the Zyxel 870HN as a bridged modem and a Netgear R7800 as router. The SPA112 works perfectly with those two as well.