Internal ring  Announces a call placed from another phone on the telephone system (internal).
External ring Announces a call incoming from the public network.
Call back ring Announces a call back has been activated and will place the call after the handset is lifted.
Confirmation Confirms acceptance of a keyed command, e.g. call back.
Dial tone Standard sound heard when handset is lifted and the line is ready to accept dialling.
Ring tone Normal tone heard when the called phone should be ringing.
Busy tone Normal tone heard when the called phone is in use ("engaged").
Equipment busy Unusual tone heard if there are no lines currently available to the called phone.
Diverted Advises that calls to this phone are being diverted. Confirms a follow-me.
Call waiting Announces that an incoming (external) call is waiting. Injects a tone onto the line that can drop fax and modem calls.
Conference Periodic tone warning that there are more than two parties involved in the call.
Unobtainable Tone heard when there is a fault condition or the dialled number is not allocated or handset left off.

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Last revised: November 09, 2020