I see it mostly used mostly as a bad second factor implementation. It’s apparently not that hard to steal a phone number.
Discourse has some stuff built in to do that. You can track accounts with the same up address, for example, and a whole bunch of people are convinced that fake accounts are not much of a problem. If your community is much more likely to attract people who want to create fake accounts than most, some version of phone number verification could likely be implemented for something like $1000 to $5000 if you post in marketplace.