Dewey

I have used my T-Mobile PAYG on roads in the deserts near Tucson, AZ and Alamagordo, NM without problems. It doesn't work well in the mountains but neither does my wife's ATT phone.

hungryhorace

Were you using data? If so, you will not have had a 4G (HSPA+) signal in those parts. Instead, you were either on a 2G signal provided by T-Mobile or more likely by AT&T who are T-Mobile's roaming partner.

I find a 2G signal is pretty useless for anything other than calls. Even in parts of the city and surrounding areas I drive through the T-Mobile signal regularly drops back to 2G. It's piss poor.