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Update Extension. Now you can assign the the phone/device to a different extension.
OR: you can simply delete the old extension, skipping steps 2-5 above.Click on the new extension number, or add an extension.
Under "Phones / Devices", select the available device.
Important: Never assign an extension to a device that's already assigned to an extension.*Update extension.
You will need to reboot the phone for the extension number on the display to reflect the change. However, this is cosmetic; the changes have already taken effect.
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Also see the following article. This is is useful if the phone will not update the extension number on the LCD:
How to: Change Extension Numbers