Lucent Technologies PARTNER Advanced Communications System Programming And Use Instructions page 296

If you hear two beeps, you have reached a busy system phone with
n
Voice Interrupt On Busy Assigned. You can speak after the beeps.
Be aware that the third party to whom the recipient is speaking
probably also will hear the two beeps and the faint sound of your
voice.Be brief and discreet in your interruption.
If the recipient has a Voice Interrupt On Busy Talk-Back button, wait
for a response.
If you hear ringing, you have reached an idle standard, MLC-6, or
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TransTalk 9000-series phone, or a busy system phone with Voice
Interrupt On Busy Not Assigned. Your call is not completed until
someone answers.
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Feature Reference