MetroPCS is the USA's 5th largest standalone facilities-based cell phone carrier, and they run their own CDMA and LTE networks.
Metro is in 14 markets, covering many major cities like New York, LA, Dallas, Atlanta, Miami, San Fran, etc.
They have their own towers, their own cell site transmitters (over 11,000 at last count). They beat Verizon in having an LTE network and had TWO LTE phones before Verizon had any. Verizon, of course, was the SECOND carrier to have an LTE network in the USA. Metro was the first.
Metro actually had the first LTE Android in the entire world -- the Galaxy Samsung Indulge.
OUTSIDE of MetroPCS's 14 markets, they do have "nationwide" CDMA roaming through agreements with Sprint and Verizon. Sprint is free, included in the cost of the monthly fee but it's only 1x. Verizon's is called "Travel Talk" and costs extra money, if you use it.
Again this is ROAMING when not in Metro's home coverage areas. In Metro's home coverage areas you will be on Metro's network which has over 11,000 cell sites owned by MetroPCS.
See the dark purple areas? MetroPCS owns the towers/cell sites in those areas (metro markets).
The "Extended Home Area" and "Travel Talk" are the "roaming" areas (like on vacation or business) -- but Metro does NOT NOT sell service to people who live in those roaming areas. You can only buy service if you have an address in the dark purple areas. If you stay a long time in the roaming areas (a few weeks), Metro will actually cancel your account. So, Metro does NOT really want to to use another carrier's service (for an extended period of time). Because Metro has their own service. The roaming agreements are for costumer's convenience, not for abuse.