Let me guess: calling from a cell phone?
Cellular voice codecs are designed to handle exactly that; voice. So when you call a number and you get that music track that the business thought was so catchy (or that they were too lazy to replace with something else), the noise cancelling algorithms get to work trying to cancel out the music and emphasize the vocal frequencies.
Hence, you get that weird compression noise and pockets of silence like you're trying to listen to a RealPlayer audio stream circa 1999.
-James
Current beast: E550 Coupe (M278)
Previous beasts: AM Vantage, E90 335i (modded + JB4 Map2), E39 M5