Have a look at this article: https://discuss.grapheneos.org/d/2892-voip-run-down
Also search for words, VOIP and SIP on this forum and see if any useful info comes your way.
What you want is what I was looking for as well, but I haven't found a good solution. The problem with VOIP numbers on your phone is that you still need a sim card with data to use them. The sim is naturally "cell tower tracked" and you location known to the telecom company (Unless if you use your voip only from wifi, which is not practical for normal use). You will also need a "SIP App" to use the VOIP. The app needs some kind of notification service to work like google play, and its likely the app will gather personal data on you like other corporate apps do. The better quality "apps" are paid apps (still not private though) and to pay for them you might probably have to use google play pay method. Another point of privacy loss.
As for a well priced sip service I know of voip.ms . Have a look. There might be better ones.
As for 2FA using a phone number, it's very likely that no voip number will work for major companies like google/microsoft/amazon/etc. These company can detect voip numbers and prevent their use for 2FA. Smaller less capable companies might accept voip numbers. But the big players don't. So generally you may not be able to use your voip number for 2FA.
Benefit of voip numbers from a decent voip company is that you can use it worldwide when you travel, and you can use it on different devices.