When your iPhone drops calls left and right even with a strong signal indicated, there are a few things you can do to correct the problem:
1. Turn your iPhone off once a day for 30 seconds. This will reset its communication with cell towers in the area.
2. Turn off 3G service and use EDGE. Settings -> General -> Network -> Enable 3G. Users have experienced more voice stability with EDGE in some areas.
3. Call your service provider. They will know if cell towers are degraded in your area. Your carrier can also file a report for network technicians to investigate.