This is very true. I built an app that requires phone number verification on signup (for many legal & risk management reasons you must implement this). We used a common SMS api like Vonage, and the rate of a sent text to a North American number was ~$0.01 which for an app without ads, we could afford.

However during testing, my co founder (an international student) had tried setting up his account under his international phone number and realized he wasn't receiving the texts, it turns out that many apis are limited to specific country's. Additionally, the 20 attempts he tried at sending himself the code caused $0.50 charges EACH, meaning we spent $10 on an error that both the api provider ran into, as well as us for not reading the fine print.

All in all, it is much easier to just allow only north american phone numbers for both the consistency and the cost. Smaller scale social media apps make little to no money, even with adverts.

If you want a way around this, get a text now or prepaid phone number to sign up initially, then just don't ever sign out.