This is the key. With Okta for example, you can configure a maximum velocity for a user for Behavior Detection to trigger additional scrutiny on a login. That type of stuff is useful.
I am not a network guy at a cellular company, but IMO it would make more sense to use a more local gateway for outgoing connections rather than potentially routing things all the way across the country. These days people keep their number but move all over the country. It would be insane to have to route things back home every time.
Checking geo IP services on my phone usually put me in roughly the same metro area that I'm physically in despite my area code belonging to a city hundreds of miles away. That said, I just tried a lookup on the cellular network on Maxmind and it thought I was in the next state over (a couple hundred miles off).
IP geolocation services usually aren't as great as what people think. My residential home IP had probably previously belonged to some Canadian ISP as things that would base their defaults off a detected geo IP lookup would think I was in some small town in Quebec despite living a thousand miles away. IP addresses change hands, people connect through all kinds of proxies and CGNAT gateways, location databases get old.
I'm not basing it on my phone number that's for central NC and from 10 years ago, I'm going off of the geoip and the fact that I get tons of ads or sites defaulting to Atlanta for weather or local store searches if I don't allow them more device based location data.