How to answer phone calls in another phone via Bluetooth? I have two Android phones which I wanted to set up one as the one that answers calls for the second that could stay in the backpack. Is it possible to use a Android phone as a Bleutooth accesory, like a hands-free device?
According to the answer to this Stack Overflow question: you can't. (It is possible for land lines though.)

Or you can forward your calls from one phone to the other, either by changing the settings in your phone or downloading an app that does it all for you.

The alternative solution that comes closest to what you're after is to install mysms mirror, which is an app that tells you when your other phone receives a call or text message, and lets you call back.  See their blog.

Google Voice might also work for you. It's mentioned in the answer to this question: Answer my phone on my Tablet?