I have a /29 range of public IP addresses assigned to me by my ISP. I'm trying to configure a SIP VOIP handset to register with my VOIP provider who recommend using public IP addresses rather than NAT.

I have a DrayTek 2820 router flashed with the latest firmware and have configured my router as per DrayTek's FAQ at:

How do I use a public subnet on the LAN (non-NAT operation ) ?

My IP range is:

xx.xx.94.16 -> xx.xx.94.23

This gives a usable range of:

xx.xx.94.17 -> xx.xx.94.22

My router's public IP address is: xx.xx.94.17, the SIP VOIP handset is allocated xx.xx.94.18.

I have a second internet connection and via that I can ping the handset.

However for some reason I can't seem to get it to register with the provider.

I tried adding a new Firewall filter to pass through from WAN to LAN:

Source: ANY, Destination: xx.xx.94.18, UDP - Ports 1024 -> 65535

Out of interest I also tried opening port 80 to see if I could browse to the phone's admin web interface but no joy.

I know that my ISP aren't blocking inbound service ports because I NAT Port Forwarded port 80 to one of my internal web servers and it rendered a test page I had set up.

All the NAT settings are reset to factory defaults, i.e. there are no Port Redirection, DMZ Host, Open Ports or Address Mappings configured.

The handset I'm using is a GrandStream GXP-2000.

Is there anything else I should be doing?