A little background first:
Currently, we have a main office and a remote office. They are connected via site-to-site VPN. Main office uses a Cisco router with DHCP at 10.10.10.0/24 while the remote office also has a Cisco router issuing IPs at 10.10.11.0/24.
Our phone system consists of the AT&T Syn248 gateway and its corresponding IP phones. The gateway is what connects our regular phone lines to the network switch and has it's own IP address. The phones connect to the network via Ethernet and are assigned their own IP address via DHCP. Only the main office has a gateway but both offices have the IP phones.
In this context we can say the devices have the following IP addresses:
Phone gateway: 10.10.10.5
IP phone: 10.10.10.6
IP Phone: 10.10.11.200
The issue I'm getting is that the remote office's phones cannot connect to the main office's gateway. I've tried to set the gateway on the remote phone to 10.10.10.1 to see if that would do anything but I can't seem to pick up the main office's phone gateway. I've also gave it a shot in the dark and set a static IP on the phone so that it is under the same IP range as the main office but that didn't work either. I've also taken a look at IP forwarding on the Cisco routers but am unsure if doing so would work as well.
Is there any way to get the remote office's phones to connect to the main office's gateway? Any help would be greatly appreciated!