Not free, but it could get around signing up on a service that forces a non-VoIP number, but lets you change it later with less strict enforcement.
Like spambait throwaway email accounts, but visible to the world indefinitely. Shouldn't use with any service that allows account reset through SMS alone, or who has your real identity or payment info. Think: logins you would put on BugMeNot
Also:
https://sms-hero.com
https://textverified.com/free
[other paid services exist]
formerly used the Ring.To system
RCS is planned for the future, but awaiting more public API support.
$5/month as of 2023
The SMS option (playSMS.org) might be a starting point for developing a zero-trust 2-factor authentication gateway.
Cheaper than Callcentric and Les.net, and supports SMS with app