Providing mobile network coverage in a country that is as far stretched as United States is not easy. It’s so difficult that American operators are working under an agreement whereby they share antennas to ensure that every operator is able to provide connectivity to its customers, whether or not it has its own antennas in the region. This is called “Domestic Data Roaming”.
It’s an option that major operators like T-Mobile, AT&T, Verizon and Sprint propose to their customers. It allows their customers under contract (Americans) to access another operator’s network if and when their network is not giving enough coverage in the area where they are situated. Operators using this service pay a hefty bill to the service providing operator but generally it’s free for domestic and international customers (with foreign SIMs). The service is very limited though. It allows only a few tens to hundreds of Mega-bytes (MB) data in 3G and sometimes international calls are prohibited.
For a tourist, it’s very inconvenient. The phone does not lose network but the services are very slow.
This option is not available with virtual operators like Lyca Mobile, Simple Mobile or H2O that have access only to the personal antennas of those operators with which they have signed an agreement. This explains why sometimes users do not receive signal in some of the most remote parts of United States. Read the post About coverage in Page, Arizona.