Audio continually turns down when telephone activates
 

I have a 2007 221 (S500/S50) and the radio or if a CD is being played all of a sudden goes silent for around 3 or 4 seconds each time the system activates the telephone. I know it's doing this as a message comes up on the screen.

It appears to be quite random and happens around a dozen times per hour. Sometimes 3 or 4 times in a row and then 30 minutes without any issue.

Does anyone know how to stop this continual interruption?

Thanks