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.
•Interrupted dial tone, also called broken or stutter dial tone, is heard when you are either forwarded to another extension or have do not disturb enabled.