ZyXEL Communications ISG50-ISDN Application Note page 13

If "Attached Voice File" is selected, you can listen to the voice message by downloading the attached file in the notification mail.
If the voice message is not attached in the notification mail, you can dial the feature code and the extension number to hear the voice message.
The default feature code is **. In this example, you can dial **1006 to hear the voice mail of extension 1006.
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