If near monopolies agree to rules sets with one another, they can effectively monopolize. That’s why there are regulations in place to prevent that behavior but we’re consistently seeing the lack of enforcement of those rules. Sure there are still other telecoms other than these two, but in the US each of the major telecoms are guilty of this sort of behavior, and while phone unlock is allowed they create unnecessary barriers to make it more difficult for consumers to do this, at the benefit of themselves. It’s similar malicious compliance to providing an ability to cancel a subscription but making it difficult to do so for consumers so they give up trying.