This can be done by going to "app info" for default OnePlus phone app and disabling, then by going to Google Phone "app info" and clicking on Phone App, then on the "Configure apps screen, changing the default Phone App to the only option you should have (after disabling the default one) which is the Google "Phone". Then make sure your new Phone app has all it's permissions.
Update: TWRP install method added for those not using Magisk. Just flash the zip and good to go.
Magisk Module v2.1a: Update Sep 5, 2017