T-Mobile plans to start utilizing mobile cell sites traveling at 17,000 mph to boost coverage before 2023.  

At an event hosted at SpaceX's Starbase facility in Boca Chica, Texas, T-Mobile's Mike Sievert and SpaceX's Elon Musk unveiled a "Coverage Above and Beyond" agreement with SpaceX on Thursday night.

SpaceX's constellation of low-Earth-orbit Starlink satellites will deliver messaging, eventually voice and data, for T-Mobile. 

Sievert said that the partnership envisions solving the 'biggest pain point in the over-40-year history of our industry' - mobile dead zones.  

Future Starlink satellites will use a mid-band spectrum, which is suitable for most headsets in the market.  It is a block from 1.91 to 1.995 GHz, which has yet to be used for a 5G network. 

Sievert aims to have most phones able to operate immediately away by using a spectrum that their phones already know.

Towards the end of 2023, a beta test of Starlink-routed messaging is planned to take place in selected areas in the U.S. and its territorial waters, including Hawaii, Puerto Rico, and parts of Alaska.

For international customers, T-Mobile would allow roaming if other countries' carriers reciprocated.  

The messaging support would include "SMS, MMS, and participating messaging apps," which may include Apple's iMessage and Google-backed RCS, as well as Signal, Messenger, and WhatsApp.

All these will depend on the message providers' cooperation in separating messaging traffic and operating in a payload-aware manner.

Musk presented that one of the huge benefits of the messaging service is redundancy, emphasizing that your phone would still work even if all cell towers were taken down. 

The service's utility during extreme weather values Starlink's added resilience.  

Musk warned that early in the service's introduction, customers waiting for a compatible Starlink to pass overhead before sending a message could happen.

T-Mobile's service will need upgrading existing Starlinks to launch the Falcon 9 and the Starship.

SpaceX has been trying to broaden Starlink's economic model beyond offering $110/month broadband to hard-to-reach homes.

It has begun selling services to recreational RVs for $135 a month and maritime services at $5,000 a month.

Also, plans of providing inflight WiFi via Starlink on Hawaiian Airlines and JSX are now lined up.

Musk continues to remain conservative in his suggestions, although he seemed to have been looking forward to the future endeavors of his ongoing partnership with T-Mobile.