I used that $35 Pioneer unit with two of my Premier headunits. It was actually really nice to have and definitely worth the measely $35 it cost. No matter where your phone was in the car, any time you have an incoming call or made a call it would mute the stereo.....so it was very very rarely that you ever missed a call due to your stereo blaring //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif And since it is a wireless unit, it would also work with anybody's phone in the car. If you are truckin' around 3 buddies, and one of them has an incoming call on their cell phone, your stereo will mute.
Only problem I had was that, since it is wireless and picks up any signals in the air, depending on where you are it can mute for apparently no reason. There is this one road near me that apparently had some type of transmitter near it.....because it would always always mute during a certain stretch of that road. However, that Pio unit does have an on/off switch (so you can turn it off in situations like that), as well as a sensitivity switch (high/mid/low).
Also a word of warning; while the stereo is muted, turn down your stereo if you had it up high. Because as soon as you hang up, your stereo will unmute.....and if you had it blaring when it muted, it will be blaring when it unmutes and scare the living shit out of ya (happened to me a few times //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif )