All of the following can/will affect the quality of the call you experience:
1. The radio frequency your carrier is licensed to use. In the USA the older cellular systems were at/around 800 Megahertz, but newer carriers (sprint, part of Verizon, part of what is now cingular) are at 1900 Megahertz. Frequency affects propagation, which is the ability of the signals to penetrate fixed objects and to carry over distances. Lower frequencies = better propagation.
2. Where you are in a cell sector relative to the tower can affect the quality of your signal. Closer = better. Coverage area can also be affected by the number of active users in the cell at any point in time, depending of which technology the carrier is using.
3. Some phones/manufacturers/models just seem to have better radio electronics than others. Could be a quality problem with an individual phone, or could be a problem with an entire model line one year for a given manufacturer
4. Overloading of a cell sector with active users can degrade the signal to noise ratio and result in dropped calls and poor reception.