How can I get the "Phone button to work in a Sony car audio system?

I have a Sony Audio system in a Ford Focus which I recently bought. I know how to pair this with my mobile phone. However the first step is to press the "Phone " button which allows me into the Menu which should say something like "Phone not connected"
However when I press the phone button it simply mutes the audio sound. The small green and red symbols also do the same thing. Is there someway that I can get the Phone button to work.