There's several options/techniques for doing it. But just imagine you have a permanent zero overhead VPN.

I don't know if that provider terminates long running calls, but the calls would stay up too regardless of tower.

Yes, I’m sure it is iOS anti-tracking and directly related to why firewall apps inside SIP my not know what is going on.

More likely to be just standard Mobile IP https://en.wikipedia.org/wiki/Mobile_IP. Fairly standard stuff, can cause some false positives around traveling (I've seen people get freaked out about stuff like "This person just logged in from their home state and then less then an hour later logged in from France!" when it was just mobile IP treating their phone as still in the US while they were in France on a trip, but their laptop connected over normal internet was seen as coming from France)

this was a consistent ip address nothing to do with location and nobody was freaked out.