Switching from active call to a waiting call? I'd like to ask you if you know how can I change the line of calling. For example, a friend of mine is calling me and we have a talk. Meanwhile, another one is calling me and at this moment I'd like to start conversation with the second one and to put on hold the first one.
I guess there should be an app for this?
Thank you in advance.
You don't need and App to do this. This is the basic "Call Waiting" feature.
Go to:

Settings -> Call Settings -> Additional settings -> Enable Call Waiting