I have an iPhone that I use mainly to connect to internet and another device that simply makes calls and not have much features. It becomes frustrating using two devices when answering calls. There is a feature called "Call Divert" which charges for each call diverted, and using a Bluetooth headset is inevitable especially when you are having much less call frequency. So is there any app, without the need to be JAILBROKEN to connect the iPhone with other Bluetooth enabled device and simply act as a sound pathway so that calls and music on the other phone can be heard on this iPhone. More like using an iPhone as Bluetooth headset.
Yes, on your iPad or iPhone go to settings > facetime > choose the caller id option for your phone number, then an option will appear to allow iphone cellular calls.