Can I use Android tablet as a handsfree or headset Bluetooth device profile? (Status in 2018)

Question

This question is a duplicate of Can an Android phone be used as a Bluetooth headset/speakers? from 2011. I am asking because I suspect Android may have been subject to an update or two since then.

My use case is:

• Old car with aux input
• Tablet screwed into the dashboard
• Tablet audio out into the car radio and wired microphone next to the rearview mirror into the tablet

I want to get in my car and take a call, without taking my phone out of my pockets. I've been searching an hour, reading 25-page long forum threads, still no definitive answers and almost all threads are from 2012 or so.

With all the people screwing tablets into their car's dashboard, there just has to be a way to do this, right?

Answer 1

Both sides of Bluetooth handsfree profile (they are called handsfree unit, HF and audio gateway, AG) are implemented and present in Android source code. As a developer, you can develop firmware supporting HF, or AG, or even both (automatically or manually selecting what role will currently be active). But generally, firmware developed for smartphones and tablets supports only AG role and firmware developed for car head units supports only HF role.

I have not tried to search such information, but possibly alternative firmwares for specific tablets with dual roles support exist. There are no serious difficulties to implement it at a firmware level. But I am not sure that the implementation of standard second role at the application level is possible.
Upd: It can be difficult to support dual roles if audio streams are processed and routed by hardware, and there is no default streaming for one of roles.