I use Zen DV on an FTTP connection via a Draytek 2960 with a FreePBX switch - I found this page useful regarding which ports/IPs need forwarding:

https://james-batchelor.com/index.php/2024/11/01/zen...

I've had it 18 months and it's worked flawlessly. My employer also uses Zen DV on a 2860 which I set up and it works without issue, once the inbound ports are forwarded correctly the outbound NAT takes care of itself.

Thank you for this. I'd come across that website and used its suggestions, sadly, without success. Perhaps some anomaly with the Gigaset phone that does not play nice.

Ken

Sometimes you get equipment that just won't play ball no matter what you do, good luck with it anyway.

Yes, I found port numbers critical, as noted on here:
https://www.aligrant.com/web/blog/2025-11-12_zen_sip...

Zen has a half-arsed implementation which will only really work with a single device on your network. I assume the FritzBox has a SIP broker built-in that will redistribute inbound calls to multiple handsets.

You absolutely must ensure that you're using outbound static port mapping on your NAT configuration. I've done this on pfsense by fixing anything destined to UDP/5060 getting a static translation - but you could also do it based on the internal address of your phone etc. I think the obvious impact of this is also you can only have one port in use on an IP address at any given time, so if you're static mapping port 5060 from your external IP, then if you have multiple handsets trying to do that, I expect you'll hit problems.

I've attached one of these: https://www.amazon.co.uk/Adapter-Protocol-Broadband-... and configured as per the link above to Zen's servers and it works fine with my old fashioned wired phone.

The slick answer is to run a SIP gateway/broker on your router (or wherever) that is the only thing that communicates with Zen VOIP and your phones register with that. If Zen supported SIP fully, it would respect the NAT port number that is embedded in the SIP registration packets, but Zen just ignores that and there's naff all you can do about that.