AT&T 7406BIS - Manual (Page 64)

Tones and Their Meanings
Ringing tones are produced by an incoming call. Feedback tones are those that you
hear through the handset (receiver) or the speaker, if you have one.
Ringing Tones
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1 ring — A call from another extension.
2 rings — A call from outside or from the attendant.
3 rings — A priority call from another extension, or a call from an Automatic
Callback call that you placed.
half ring — A call redirected from your voice terminal to another because Send All
Calls or Call Forwarding — Follow Me is active.
intercom ring — A call directed to you by the Intercom feature.
Feedback Tones
● busy tone — A low-pitched tone repeated 60 times a minute; indicates that the
number dialed is in use.