In order to be able to listen in to a call you will need to meet the following prerequisites
The PBX will need to be a Professional or Enterprise license.
Both extensions will need to be in the same extension group
The extension which will be listened in will need to show their extensions presence, and active calls
The extension which will be listening in to the above extension will need to see the extension, and its active calls
Finally, the extension which will be listening in to the extension will need to have the relevant group right (or individually at the extension level)
The person listening in will need to be using a 3CX App for Windows or the 3CX Web Client
Then, from the switchboard of the 3CX Web Client, or 3CX App for Windows, right click on the call you want to listen in to, and select "Listen"
The Client of the listener will show "Barge In" at this point, but the functionality will be listening, if this was the option chosen.