Lucent Technologies MLS-model Programming And Use Manual page 402

Feature Reference
3. Respond to the originator of the voice interrupt on busy call.
The third party with whom you were originally speaking does not hear your
response, but you can hear that third party.
4. Press the Voice Interrupt On Busy Talk-Back button again (whether or not
the originator has hung up) to resume the conversation with the third party.
The green light next to the Voice Interrupt On Busy Talk-Back button turns
off. Voice Interrupt On Busy Talk-Back is deactivated. The originator hears
two beeps if he or she did not hang up previously.
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