Your service provider is usually not the primary reasons for service problems. That resides almost solely on your choice of phones.
Service providers "rent" airtime from other providers' towers in areas where they are not the primary provider. In other words, you've got coverage. The trick is to have coverage for all the phone types. A dual-band (or better, a tri-band) phone will keep you in coverage. The better phones usually also have better descrimination and noise reduction circuitry that keeps service interruptions to a minimum and clarity at a maximum.
The "free" phone you get with your local service plan is usually a cheap piece of bovine excrement. They are the cheapest thing the company could get that would actually power up. It's a marketing tool to get you in the door and your name on a two-year contract. Nothing more. If you want a phone that works well, be prepared to ante up for a good one that will work on multiple systems.
In short, don't worry about the service provider, worry about the phone.
My father is a good example. He's had three or four different "free" phones that worked like crap on their rural cell phone network. My phone, which is an Audiovox tri-band I paid a good bit extra for, works quite well. My brother's "upgrade" Nokia also works just dandy. My mother's phone, which is my brother's previous "upgrade" Motorola is fine, too. All that, and my father is still firmly convinced that it's the provider's fault. There just no way it just couldn't be the cheap, crappy "free" phones he keeps picking. Sigh.
Brad