National roaming, this is a solved problem and just how it works in some parts of Europe.

These days many phones have radios for all networks in the US. I don’t know how much Verizon still runs its CDMA network, but for the last several years it’s not been a hardware thing.

Verizon shutdown CDMA on December 31 2022.

On LTE and 5G, phone calls are already just VoIP (VoLTE/VoNR) over a pure data network, ideally you'd decouple your VoIP service completely from the dumb data pipe.

Of course the last thing that any ISP (wired or wireless) wants to do is commoditize their service as a a dumb data pipe - they all want the margins of those over-the-top services.

Isn't it how MVNOs worked, even in 3G? The phone tries PLMNs listed in SIM, the Node B/RNC relays auth requests to HLR that corresponds to that IMSI or number, and that HLR could be anywhere so long the RNC knows the ... address?

You need to define those terms for the rest of us

Not exact translation, but:

The SIM card contains information that allows it to negotiate with available cells regarding which cells and with what parameters it can connect.

The cell relay network (NodeB/RNC/etc) the connection to their owner service which sets up authentication with the SIM card - either by consulting service's "phone number database" based on the IMSI number of the SIM card, or by forwarding it to roaming peer's database (HLR). Then it keeps tunnels between phone and appropriate gateway.

SIM provides the identity of the "account/connection".

IMEI provides the identity of the actual terminal (arguably a bit like MAC address) - called "User Equipment" in UMTS/LTE/5G

BTS/NodeB/RNC/etc. - the radio segment - BTS/NodeB/ENodeB being the "base stations" and RNC is the radio controller (also called Mobile Management Entity in newer standards) which handles setting up connectivity from UE over NodeB to appropriate gateways (connecting to IP network or circuit switched phone networks)

HLR/HSS - Home Location Register / Home Subscriber Server - the part of the system that authenticates the subscriber identified by SIM. It's what maps the connection to right entity in network (to route phone numbers, match billing, etc.) - when you use roaming, the RNC/MME is set up in a way where it can find your original HLR/HSS (tackling extra fees as necessary)

gateways - different depending on version of network as well as whether you use IMS-only network (aka VoLTE/VoNR, in actuality part of 3G known better to people here as VoIP/SIP), or whether circuit switched fallback is used (operates purely on 2G and 3G, with latter being added in later revision).

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?

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.

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