Who do we arrest if a crime is organized via phone call on T-Mobile’s network

I guarantee you, T-Mobile does not hesitate to hand over any and all data they have to the government. And they don’t encrypt shit, as evidenced by their many many data breaches.

or via mail?

The postal service is from a different era, and has legal protections I wish online equivalents had. Logically they should. Realistically they probably never will.