T-Mobile, a major phone provider, runs an ISP which is IPv6 only. That is, your phone never gets an IPv4, unless connected to WiFi. They offer home access points with a 5G modem and a router; the external address is also IPv6 only.
It works plenty well. I access everything accessible via IPv6, and the rest through their 464XLAT, transparently.
My LAN still has IPv4, because some ancient network printers don't know IPv6. OpenWRT on my router supports IPv6 just fine. Of course I do not expose any of my home devices to the public internet, except via Wireguard.
Ironically there's T-Mobile Business which is static IPv4 only.
Not here in Germany - our T-Mobile Business access only gets a static IPv6 and our main fiber uplink from Telekom (same provider) gets both.
I suspect it's an acquired property with a sufficiently separate network.
If the service area is the same, it's probably tunneled. You'd be surprised how much tunneling ISPs use. They're not connecting your network directly to their network.