"Only You" is an application which mute all calls, except those you list as important enough that they can "wake" you up! With "Only You" you define a list of "exception" numbers. When you enable the application, each incoming call from a number not in the "exception list" will be silent (no ring tone). Each call from a number in the "exception list" will ring normally. In the extreme case in which you don't have any "exception" number, the phone will be silent for each incoming call!
Take a look at it in Android Market