prompter
I believe you could still make phone calls in the default dialer app on the Pixel Tablet running GrapheneOS, by using a VoIP provider, rather than the mobile network.
I guess you could have a look if there is still a dialer and SMS app on stock Pixel Tablet OS.
As a small aside, location requests can still be made on the Pixel Tablet running GrapheneOS, even though it has no GNSS receiver, because they can be made via Google location service with Google Play Services installed instead.