Intercom

Each of the six phones must have line 1 connected to the same "subscriber loop" (the same phone number). Assuming this is the case, and that you have not assigned any numbers to the the top six speed-dial buttons, each station will have agreed to be assigned a station number.

From any station, pressing the speed-dial button for another station's station number will page that station.

Now, how to properly initiate an intercom conversation differs between announcing to another station that a call is on hold, or a "cold call".