Hi Dan and welcome.
So the 746 has been "converted" to use a modern BT plug?
And there was a linecord with an RJ11 on each end included with the LiGo?
Have you got a spare broadband microfilter lying around?
You can use it as an adapter to connect the phone to the LiGo like this,
The filter also contains a ringing capacitor, simulating a master socket, so the phone should ring on incoming calls.
Mine a different gateway device but the principle is the same.
You will not be able to dial outgoing calls on that phone as things stand, because it's most unlikely the Ligo will support pulse dialling. There are ways around this, but one thing at a time.
<<Edit>>, seems I may be wrong about that last point, I've just seen reviews saying they have used rotary dial phones with success, I must say I am both surprised and impressed.
£22.98 including shipping, not bad.