Manager and agent VS. caller

suppose that I am manager or admin and I want to talk with agent without the caller can listen, and the agent and caller can talk,but I can listen both, i.e, the caller listen to the agent but it does not listen to me, and the agent into a conference with caller and with me. That is possible? how? there is some app for that?(app_meetme don´t help me to make that).Thanks for their time and excuse me by my english.

PD: I believe that “coaching” is called
PD: It is for a call center.