This was my experience with Verizon. My coverage in my house with my first phone was awful. I couldn’t make calls from the bedrooms (on one side of the house) or the basement (which is a walk-out, so not really underground) at all. With a newer phone, I could make calls from one of the bedrooms, not the other two. That never improved.
I’m now an AT&T iPhone user. The only dropped calls I have are with my partner, who’s a Verizon user. Data speeds are good (though I’m usually on wifi) and I have no complaints about customer service. I hope that stands when I move to the DC area in the fall, (though I don’t have problems when I travel to the area) because it seems like AT&Ts biggest problems are in the largest metro areas.
But I will say this for AT&T. A few weeks after I switched, I noticed that I went from a persistent 3 bars/3G throughout my house to 1 bar/EDGE throughout my house. I called customer service and they confirmed that a tower was damaged in storms we had the week before, and that service should go back to normal ASAP. Within 3 days, I was up to persistent 4 bars/3G at home, and that’s where I am to date. They also gave me a service credit of 25% off my bill that month for the inconvenience.