Iphone 3GS not pairing to plantronics voyager 520

Without turning the headset on, turn the bluetooth on for the phone. Select the headset that is paired, and select Unpair. Then turn bluetooth off. Put the headset into the "discover" mode, by holding down the power button until the light flashes red and blue. Then turn the bluetooth on for the phone, and it should "discover" it. It should ask for the passcode, normally 0000. Once you enter that, it should pair. After that, you should be alright. You can turn the headset off and back on again and it should connect right away.
Hope this helps.
otherwise try this : http://www.plantronics.com/media/media_resources/easytopair/