chevrolux: Doesn't sound like firewall issues as registrations aren't even being sent from your device - because I assume you have a consumer router so no outbound firewall.

I've gone right off the Gigaset's because they seem to just do weird things.

To start with though put an IP address in for the proxy (for me pan.wxnz.net resolves to 64.74.223.41) instead of the domain. That will rule out DNS issues.