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