Once in Dispatch (and you're set up as a Base Station), click on the terminal you wish to open direct communications with.
In the menu that appears, look for the P2P (Peer to Peer) section, and select the option to call the terminal you selected from your base station.
The terminal of the selected user will now notify them of the incoming call, with the options to take the call by pressing 'OK' or by pressing the talk button to open communications.
To close the call, the terminal user or the operator can set themself back to 'Available'.
Alternatively, the operator can close the call from Dispatch using the same menu as the call was started from (click 'Stop').