An update: Sadly I am giving up on VOIP/SIP for now. I have been messing around with this for almost two months now and I am still having major technical issues (e.g. the other party can't hear me after a certain period of time). I Tried about 5 different apps and have been in touch with my ISP and SIP providers more times than I can count. I still don't know the root of the problem at the end (I resolved so many along the way). What I learned:
1) VOIP.ms is an SIP provider - their support is excellent. They are KYC though and they don't have their own app unfortunately.
2) Virgin Mobile technical support literally has no access to advanced settings. There is no one that can tell you Port numbers for example. Their customer service is excellent for basic things but limited.
3) Some of the apps have problems with GrapheneOS specifically around push notifications. Linphone worked the best for me. I used the independent app "VOIP.ms app" for SMS
4) Be prepared for a lot of DIY work. I was motivated and full of energy but I eventually gave up.