Finding a reliable VOIP phone app that works on GrapheneOS can be tricky. The go-to privacy phone number provider is MySudo but their app doesn't work without Google Play Services.
Cheogram (available on F-Droid) + jmp.chat seems to be a decent solution for $3 a month. You can pay with bitcoin or with a privacy.com card.
Have you tried Linphone? That's on F-Droid as well and is pretty feature rich.
That's the other recommendation I've seen. Haven't tried it yet.
have you tried Crypton? https://crypton.sh
What about silent.link
Been meaning to check them out. Not sure how their burner numbers work / if they have a VOIP app.
Been using them for about a month. Works fine so far. Cheap anon numbersandd data.
Proovl has worked well for me at least: https://www.proovl.com/
Maybe nostr:npub1vr2nva0s0hhfulthjy8053rg9kruk5erzwaxdw85gjwkfytj994sgudzvx can answer your question.
What about phone calls through signal app?
The Calls app supports VoIP, I haven't tried it yet. Available with postmarketOS, PureOS and others.
microg maybe? https://microg.org/
nostr:naddr1qqxnzd3exqmn2v3k8qmrjdpjqgs9hejyd252x8q3kw6980ud4lymx3hlx2x3lgg6p7sz58nyv8m2nvgrqsqqqa28cxfg47
