There are two different dial tones when you call with a system telephone:
•Outside dial tone is generated by your local telephone company to indicate that you are connected with an outside line.
•Intercom dial tone is generated by the system to indicate that you are connected with an inside line. You hear this dial tone when you are making an inside or intercom call.
•Four Rapid Beeps: This indicates that the phone is waiting for an external line.
•Double Beep: The feature you dialed was completed. For example switching do not disturb on or off.