AT&T 7406BIS - Manual (Page 65)

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call-waiting ringback tone — A ringback tone with a lower-pitched signal at the
end; indicates that the extension called is busy, but the called party has been given a
call-waiting tone.
confirmation tone — 3 short bursts of tone; indicates that a feature activation or
cancellation has been accepted.
coverage tone — 1 long burst of tone; indicates that your call will be sent to another
extension to be answered by a covering user.
dial tone — A continuous tone; indicates that dialing can begin.
intercept/time-out tone — An alternating high and low tone; indicates a dialing
error, a denial of the requested service, or a failure to dial within a preset interval
(usually 10 seconds) after lifting the handset or dialing the previous digit.
recall dial tone — 3 short bursts of tone followed by a steady dial tone; indicates
that the feature request has been accepted and dialing can begin.
reorder tone — A fast-busy tone repeated 120 times a minute; indicates that all
trunks are busy.
ringback tone — A low-pitched tone repeated 15 times a minute; indicates that the
number dialed is being rung.