If the user has changed phones (even keeping the same number) or has for instance begun using 5G (which wasn't available 5 years ago...tmo started rolling it out at the tail end of 2019), they're using a materially different product than they were originally contracted for, and have likely agreed to new terms. Not saying that isn't scammy or scummy, but just like you've changed the product you're using, they can change the product they're selling.
In Washington state electric vehicle owners pay a large surcharge on our annual car registration fees, meant to replace the gas taxes we're not generating.
It is functionally trivial to rotate IPs from the "test phones". There are shady brokers who will sell you Residential IP-based proxy servers (hacked home machines which will proxy your calls and make them appear to come from homes all around the world).
My company ( PushSpring ) has a product that lets you track arbitrary data and events about your mobile app users and then segment them. We also offer the ability to send targeted messages to segments you define via Push Notifications, as a way of keeping them engaged (or re-engaging groups who've left). We're working with some of the largest publishers in the app store.
Love to chat about your requirements -- [email protected] if you'd like to discuss.