T-Mobile’s requirements for supporting messaging programs on Shortcode, Longcode, and Toll-Free on the T-Mobile network are outlined in this guide:
T-Mobile Code of Conduct version 2.2
Consumer versus Non-Consumer Texting
This Code of Conduct governs the existing Short Code Application-to-Person (A2P) messaging system and the AT&T 10-digit long code (10DLC) A2P Messages. The 10DLC A2P Messages allows A2P SMS traffic originating from 10DLCs outside of AT&T’s network to be delivered to AT&T mobile customers
/att_code_of_conduct_062020.pdf
Verizon adopted a code of conduct based on the CTIA guidelines .