Best way to do this?

When a user makes a call to my pbx, he needs to enter another phonenumber. Then asterisk should call that number and when the owner of that number takes the phone, asterisk should play a sound.

The user who made the call to my pbx, should listen live to the other call, he will hear the sound played from the pbx and the response of the user.

What is the best way to do this?