Hi Lewis,
Can you retry the set-up you've mentioned. If there is no voice light please restart (powercycle) the NTD and TG797n V3. Give it a few minutes to see if the voice light comes back on.
It's possible that this set up may not work as bridging removes the control the modem had over the connection. You may need to look into getting an IPPOTS voice system, which will connect to the UNI-V port of the NTD rather than using the VOIP connection which uses the TG797.
Ben