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.