Bluetooth iPodTouch/iPad App?

Does anyone know of an app that will allow me to pair my cellphone with my ipodtouch or ipad so that i can make calls through my ipad. i'm ultimatly trying to make it so that if i mount my ipodtouch or ipad in my car, i will able to make calls handsfree my pairing it to my cellphone.

Please forgive me for asking, but....

I know there is probably a good reason, but if you already have a cellphone, why do you want to connect your iPod or iPad through your cellphone to make calls?

I'm trying to make my ipad a dashboard. so if i can accept calls through my ipad while its mounted in my car, i can also listen to mucic and use a gps all on one device.