Intercom calls are Ring-Beep, Transfered Calls are Ring-Beep-Beep, while Incoming Calls are just Ring (no beeps)
Change the lines to ring tone 8 (the only one that resembles a telephone ring signal), then the Intercom calls will sound different. Only gotcha is the first ring on blind transfers will be Intercom style Ring-Beep, then they will continue with outside style Ring-Beep-Beep