Transfer calls

I have installed FreePBX and am right now configuring FreePBX and the phones.
All of our phone are smartphones running CSipSimple.
So far it works OK.
I have read the documentation but can’t figure out how to solve a problem.

If I get a call that I want to transfer to another extension - how do I do that.
I want also to get the call back if the extension is busy or don’t answer.
Appreciate any help.