The issue is with billing and which mobile provider is responsible for the phone number. If your phone is dynamically switching between 5 carriers, does that mean you now need to have 5 billing account as well?
Without changing existing protocols, it would work like MVNOs (mobile networks without own networks) operate, where they provide or lease identity management services and gateways to rest of the phone network, with roaming links set with owners of actual physical networks
You could imagine a separate business handling payments and managing the phone numbers. Kinda how ICANN handles domain names but you end up paying a third party.