Isn't the ability to migrate phone numbers a legal requirement for carriers?
Edit: yeah it is [1]
> Phone companies are required by law to port your number out when you start service with a new carrier. According to the FCC, a company can't refuse to port your number even if you have an outstanding balance or unpaid termination fees.
[1] https://telzio.com/blog/what-to-do-when-a-carrier-refuses-to...