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.

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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.

enter image description here

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.

  • "What could the Zyxel have more than the D7800 ?" - I am not familiar with either hardware, so provide that information, I can use my vast and extensive knowledge to determine the answer to this question. "Reason for error : DNS error." - This means the FritzBox is unable to query a hostname its expects to reach. So determine the reason you cannot reach the hosname, once you do that, you would have solved your problem. – Ramhound Apr 9 '16 at 21:31
  • There is an obvious and tremendous difference: Your FritzBox is now behind another firewall (and probably NAT). SIP is not a firewall-friendly protocol. – Daniel B Apr 10 '16 at 6:58
  • Hi, thanks for your comments. Unfortunately, the Zyxel 870HN also features NAT and a firewall. It shouldn't work with the Zyxel then but it works. Anyway, I've found a solution. – Kriegalex Apr 11 '16 at 0:09
  • I'm now using a Cisco SPA112 instead of the Fritzbox and it seems to work nice with my D7800. My goal was to save some money by reusing the Fritzbox as VoIP gateway, but hey, at least I've an alternative that works. Though if anyone has an explanation of why it works with a Zyxel and not with the D7800, I'm still curious – Kriegalex Apr 11 '16 at 0:18
  • If you found a solution that works for you, consider either answering your own question (and accepting that answer after the cooldown) or removing it. – Daniel B Apr 13 '16 at 18:30

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.

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