Thanks Raj66 for checking.
If you're unable to get the voice working in the set up Shelly / Shellock advised then it may be worth looking into an IPPOTS setup.
With IPPOTs the UNI-V port on your NTD is configured to deliver voice services rather than the T-Gateway modem. This way you can have your phones service and have your ASUS connected directly to the NTD without the need for the T-Gateway.