Mainly T-Mobile towers but they are known to use pretty much any cellular tower that you are near and make a phone call using random cellular towers. But in the greater area of big cities you will most likely be using a T-Mobile tower. But I don't see thing wrong with T-Mobile. I have always like their service other that they are trying to charge me cuz I was literally couple hours late canceling my contract with them that wanted me to pay for that whole month for those late couple hrs. That was not a good experience that's it. Other than that I like this service in general