Hi @malcomfray .
You can use some other wifi router as long as it supports a VLAN ID=2 setting (not all do).
If you want to use a 3rd party voip provider but stay with TPG for internet, there are options:
-get a real voip phone and connect to LAN port on router; program phone with provider settings; probably not worth it.
-get an analog telephone adaptor (ATA), connect it to LAN port on router; connect your standard phone to ATA; program ATA with your provider details.
-get a router that has voip function built in; connect your standard phone to the phone terminal on router; program router with your provider details.
I don't know what issues you might have in either keeping whatever phone number you have now or getting a new number.
This thread has posts from various users relating to voip.