Which Telstra VOIP Service are you talking about. Are there VOIP settings on your TG797?
The SPA112 is an ATA - a phone device to ethernet adapter.
You sit it on the inside of your network. (Ethernet cable to one of the "LAN" Ports on your router. (You havent said what router you are switching to).
YOu need to open up the appropriate VOIP Ports on your new router to allow access to/from the SPA112. You then need to configure your SPA112 to whatever VSP You are using...