First, a few questions:
Have you turned on bluetooth visibility on the Phone?
Have you put the stereo in pairing mode?
Do you know the stereo's pairing code?
IF you've answered 'Yes' to all the above, then either the phone or the stereo may have a Bluetooth fault. However, if you've answered 'NO' to any of those questions, then you won't be able to pair the phone with the stereo.