You now have double nat and an inferior result why bother with the USG if you use that setup.

I would dump the voip from the isp, get an account with 2talk or hero, buy a separate ata such as the one below and just use the USG as the one and only router, you are now free to go with any isp and not worry about voice provided by them or not, just as most of us did with email 15 years ago.

https://www.pbtech.co.nz/product/NETGSM26085/Grandstream-HT801-Telephone-Adapter-Single-Port-FX

Cyril