Your phone connects to AT&T and Verizon, two of the three biggest carriers in the country. These are not budget resellers. They are the same networks that power millions of American phones every day, so the signal you get is the real thing.
Coverage is strong across cities and along main highways. You will have fast data in New York, Chicago, Miami, Las Vegas, San Francisco and Los Angeles, plus solid service on busy routes like Route 66 and Interstate 95. Verizon in particular reaches deep into rural areas and many national parks.
Signal does drop in a few wild spots. Deep canyon floors in places like Zion or parts of the Grand Canyon can go quiet. So can long empty stretches of desert in Nevada or remote forest roads. Download an offline map before you head into those areas and you will be fine.