IMPORTANT: Most of this setup will be done on your SIP phone server. The only thing we can make the NG Firewall do is point to your server for that information.
DHCP Option 66
Once your SIP phone server has been setup you can then use DHCP options to point your NG Firewall to that server.
Navigate to the interface your phone is connected to in Config > Network > Interfaces. Edit the interface, under the DHCP Configuration tab you will see DHCP Options. Add in option 66 with the IP/URL of your SIP Server.
Depending on what address your phone points to (IP or URL) the syntax can be found here.