It depends on what radio you are dealing with. As mentioned, you may need a software update for your car radio. If it's an OEM radio then you will have to check with your car dealership. If it's an after-market unit then you will need to check with the manufacturer to see if there is an update.
I Should have mentioned to remove any previous pairings. Some will only pair with one, which means it's still paired with previous phone, but you may have covered that already.