cisco 1720 with point to point T1

I have 2 offices with VOIP and a T1 point to point line. Each office has a Cisco 1720 router. I want to tie the LANs at both offices together over this T1. What would the configuration file be to send to the 1720 to allow this. Do both of the LANs need to be domains or can I leave them as workgroups? An information would be greatly appreciated.