post https://rbm.us.vibes.com/connectCustomers//testers/
Invite a mobile device to become a tester of an agent. The device must have RCS capabilities to become a tester. When a phone number is a registered tester, they are able to receive RCS messages from an agent, even if that agent is not yet launched.
T-Mobile restrictions
T-Mobile prohibits its users from becoming testers until the agent has gone through 3rd-party verification and is submitted for launch to the carrier. Inviting T-Mobile phone numbers to be testers before agent launch will return an error.