You may need to listen someone else's call. First of all be sure to be compliant with international laws and regulations and those of your country: Rumors that the Alphabet Soup is wiretapping the whole world will not shield you from a lawsuit or a criminal investigation. If you're positive you have the right to listen, FreeSWITCH has two dialplan applications to choose from: eavesdrop will allow you to listen to an arbitrary call (defined as an uuid argument to the app), while userspy will constantly eavesdrop on calls involving a specific user.

Using eavesdrop on a call (also known as call barging) requires knowing its uuid (you may use all as uuid, but you'll end up listening to all existing calls mixed together). One such technique is implemented in the standard dialplan. When a call is processed, its uuid is added to a spymap db table, indexed on extension. You can then dial a prefix + extension, and if there is a call involving that extension the uuid will be retrieved...