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Most USA phone plans cover the entire USA, so no domestic roaming countrywide and usually no long distance charges for calls within the USA. The phone company websites usually have coverage maps available online, so you can see where you might not have access. The blank spots are often areas with simply no coverage, but some maybe served by a small local provider.
Upvote:1
I assume you like to use your phone from abroad.
T-Mobile USA uses a rather strange UMTS frequency bands (1700 MHz AWS). This band isn't supported by most mobile phones. American LTE Bands are generally different than those of the rest of the world. 2G GSM will work with T-Mobile and AT&T.
The only network that supports nationswide UMTS on 2100 MHz (as used in India) is AT&T. The LTE capabilities depends on your phone.
You can check which provider works when you arrive. Search with your phone for network operators. It will list all providers and the supported networks (2G, 3G, 4G).
All plans I used covered the whole USA and even Puerto Rico. So no domnestic roaming.