Such device already exists: it is a smartphone! As a beginner it will take you years until you will have skill and knowledge to make something better. I think you have two options: either get an older smartphone (the old ones used to be very small) and remove everything you don't need (such as the display). If this does not work postpone this project and start with something smaller to learn basics of electronics and programming - blinking a LED, etc. I think you need really understand most parts of your project. Without such knowledge you can only build something from shields and modules, using libraries written by someone else. But such project will be power hungry and bulky, probably much worse than a stripped smartphone.
the device in the photo isn't a smartphone....
Why is the word "stealth" in the Title. At best that suggests anti-social behaviour that is intended to do something without another person being aware that his call is being "stolen".
Projects like that should not get help here.
"that is intended to do something without another person being aware that his call is being "stolen""
what are talking about the purpose of the device as simple as i written in the description ...
1- someone calls my device
2- the device receives the call and answers it automatically
3- the device transfers the voice to the bluetooth earphones ...and transfers my voice to the calling device
the purpose of this device is making calls without the need of SIM and without using a smartphone at the receiving side ...