Recaller is an awesome app, I thought to use it as voicemail, if I miss call(or i dont wanna answer) it will say something like, please leave the message after the beep.
Another question, why it only records callers voice but not mine?

Probably a bug. It used to work ok. I will take another look at it in the next days (also to fix the bug that recaller hangs/stops if your save folder contains a space)

About the voicemail: I don't know if the necessary APIs are available to get off-hook programmatically without routing the audio through the system.