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You can use HUD to monitor/barge, or dialing *66 (*MO) plus a valid extension number will allow you to monitor the call that extension is making or the next call that extension makes. The users on the call will not be able to hear you. You must have the HUD - Barge permission over the extension you're monitoring.
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Select this option (you must have permission!) and the Barge function executes as with the explanation above.
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