call recorder

Maybe these links can help you:

android - MediaRecorder - record calls application - Stack Overflow

Raw Audio Manipulation in Android - European Developer Blog

How can I record voice and record Call in Android? - Stack Overflow

 

There are many call recorders already. Are you intending to add anything that they don't already provide?

A word of warning. The apps didn't work properly on mine, so I did researched a bit. It seems that many devices don't come with drivers that enable proper call recording. I found out that for those devices (including mine, a Nexus 5), either the speaker needs to be enabled (not suitable for private conversations or public locations) or you need to root your phone and install better drivers (not suitable for me). Also, many devices don't support recording from hands-free devices such as Bluetooth.

You may find this thread on XDA Developers, especially the first post, useful.