Before I bought my new Netgear D7800, I had a Fritzbox 7490 provided by my swiss ISP Sunrise (see image case A). It does all tasks : modem, router, connects landline phone to be used as IP phone. Because ISP refused to know and help about third party modems and provide support, I borrowed a friend's Zyxel 870HN that is specially made for swiss ISP Swisscom to test the connection before buying anything (case B). In that scenario, Zyxel takes the modem/router tasks and the Fritzbox becomes only a "gateway" for the landline phone.
It was a good surprise to see that with standard factory settings, it was getting a DSL signal from Sunrise servers and that I could still use the landline phone that was connected to my Fritzbox in a special "client IP mode". I was then pretty confident I could use third party modems.
However, when switching to the D7800 in place of the Zyxel 870HN (case C), one problem appeared :
Phone calls cannot be received or made with the D7800 as modem/router (it is ok with Zyxel).
When calling from the landline, it rings as occupied line and from the outside to the landline it says something like "this number is not reachable/doesn't exist".
Does anyone have an idea ? What could the Zyxel have more than the D7800 ? Or maybe what does it miss (some security feature, ...) Would a Cisco SPA112 change anything or would it be the same problem ?
There is an interesting error message on the Fritzbox : Registration of internet telephone number ABCDEF failed. Reason for error : DNS error.
I tried to disable SIP Application Layer Gateway (SIP ALG) in the router settings, didn't see any change.