You ask a great question, Caroline, and it's a query I often get from readers. A lot of variables affect a handset's call quality, but you're correct that both the phone itself and the carrier's network make a difference. On the handset side, the strength of the antenna and the receiver play a big role, while the quality of the earpiece (volume level and so on) is a determining factor as well. Not all phone models have the same innards, so some handsets can perform badly regardless of the carrier.
The carrier's network also involves a lot of variables, but they can be more difficult to rate effectively. Beyond the obvious--the location of towers and the breadth of the carrier's coverage--your location also makes a big difference. For example, while Cingular may have comprehensive coverage in a city such as San Francisco, it is possible for two people just blocks away to have different coverage experiences. Geographical factors such as hills can be responsible for the variations, but buildings and the materials they're made out of also matter. Cellular signals pass more easily through glass and wood than they do metal and concrete, for instance. Also, your coverage inside a building is affected by which bands your phone uses. In general, the lower-frequency bands penetrate through buildings more easily than higher frequencies. But wait, there are more factors: atmospheric interference, as well as how many other subscribers are making calls at a given time. The more people are calling, the harder it is to get a signal. That's why it can be so hard to make a call at a concert or an outdoor festival.
For these reasons, it's difficult to uphold one particular cell phone as the best overall, especially since the market changes so often. It's also tough for CNET to rate one carrier as the best overall. To do so, you'd have to make test calls all around the country with many types of cell phones throughout the day. Unfortunately, CNET doesn't have those kinds of resources.