Connecting remote SIP phones to the TDxxxx VoIP phone system might impose a security threat. Although there are few ways to get around that, the best way is to use VPN when possible. VPN allows you to connect the remote phones without creating any holes in your firewall. Simply configure the phone in the same way you would if it was in your internal subnet. Edit sip_general_custom.conf to reflect both the local network and the remote network as well.

localnet = 192.168.1.0/255.255.255.0   – local subnet

localnet = 192.168.20.0/255.255.255.0  – remote subnet

That’s all there is to it. Now your SIP phone will register on the TDxxxx VoIP server without poking any holes in the firewall.