Is it possible to connect an android device as Bluetooth hands-free to another device? I want to create an android app that can send voice via headset option in voice calls.
Without going into much details, It's not possible for mobile devices to act as a Hands Free device currently as per the stack of Android. The reason is, the handsfree profile is based on server client format. One device will be server which will indicate call coming, phone ringing and other commands to client device. Mobile devices acts as Server(Audio Gateway in terms of Bluetooth) and Hands Free devices are client.
So, the case which you want to implement is not possible right now unless you make the relevant change in Bluetooth stack.
Regards