FYI i figured out why mine is not doing voice dialing

My iPhone 3g does not support it.

You need a phone that supports voice dialing.

When i use my wifes iPhone 3gs which supports it, it works.

Personally i think i would prefer the mute option as i very rarely
use voice dialing as i have full access to the phone book anyway.
Its often handy to mute the radio quickly which i cannot do now.