No sounds when I get a phone calls

I have a Iphone 4 running IOS 5.1.1 and have recently paired my phone with my in car bluetooth handsfree system, which works perfectly but when I take the phone out of the car I get no sounds when some one calls or SMS's me. The speakers are working as when I play music the sound comes out no problem the slider on the side is in the non- muted position and the sound level in the settings is turned up, could anyone shed any light on this problem on how to solve it as I would like to keep the phone paired with the car hands free system.