Even the bench marks don’t seem all that helpful.

Plenty of free VOIP services exist, including SMS reception.

Even when the free service providers are manually blocklisted, one-time validations can be defeated with private numbers on real networks / providers for under a dollar per validation, and repeated ongoing validations can be performed with rented private numbers on real networks / providers for under ten dollars per month.

The rent-an-SMS services that enable this are accessible through a web interface that allows connections from tor, vpns, etc - there is no guarantee that the telecom provider's location records of the IMEI tied to that phone number is anywhere close to the end user's real geographic location, so this isn't even helpful for law enforcement purposes where they can subpoena telecom provider records.

This "phone number required" practice exists for one primary reason: for businesses to track non-fraudulent users, data mine their non-fraudulent users, and monetize the correlated personal information of non-fraudulent users without true informed consent (almost nobody reads ToS's, but many would object to these invasive practices if given a dialogue box that let them accept or decline the privacy infringements but still allowed the user to use the business' service either way).

Sometimes, they are also used for a secondary reason: to allow the business to cheap out on developer costs by cutting corners on proper, secure MFA validation. No need to implement modern, secure passkeys or RFC-compliant TOTP MFA, FIDO2, U2F when you can just put your users in harm's way by pretending that SMS is a secure channel, rather than easily compromised by even common criminals with SS7 attacks, which are not relegated to nation-state actors like they once were.