Apparently, the Motorola Atrix 4G is having some voice-quality calling issues. Motorola themselves have indicated the problem on their support forums, and are investigating. Furthermore, they seem to be working on a possible software update to fix the issue, but have to do some more testing with customers to iron it out. In fact, they are accepting 1000 volunteers to help them test. If you would like to help out, check out the links below.

Source: Motorola Support Forums - Voice Quality Issues
Source: Motorola Support Forums - Software Update News