I finally got a Moto Razr phone to use HF. I love it. My question (in a technical sense) is how does HF work? It seem that the caller from the speakers would hear their own voices?????

 

Most bluetooth systems have a noise cancelling feature - meaning that whatever sound it puts out through the speakers, the mic will not pickup. Perhaps check wikipedia for a detailed explanation, there may be one

 

Every useful handfree system (bt or not) has a feature that filters out the incoming signal to avoid feedback and echo. Microphones attached to amplifiers need this too but it's less sophisticated, typically just a filter that takes out the resonant frequency that it would screech at if it wasn't present.

If you've ever used handsfree telephones in the office or at home you know that this is common technology although the quality varies, as with many things.

 

They use noise and echo cancelling microphones. Most microphones pick up not only your voice but also any background noise, including the voice of the person you’re talking to coming out of the speakers. Echo-cancellation solves this problem by programming the microphone to track the voice of the person speaking and minimize or eliminate any background noise.