The problem is many services out there using SMS for authentication codes blacklist VOIP numbers and only accept real phone numbers tied to a SIM/eSIM. E.g. In USA, the Social Security website doesn't accept VOIP numbers to verify logins. Also, some software purchase validation schemes (i.e. using phones as a form of "DRM" restriction) reject VOIP numbers.

In the early 2000s when VOIP started being offered to consumers, nobody checked for "is it VOIP?" so didn't rejected them. It was as good as a real phone number. But that has changed and more and more places will not accept them. A lot of financial services (trading platforms and KYC reject VOIP numbers).