Shared Extension is when a group of agents has one and the same extension line. During the incoming call the phone will ring at several agents simultaneously.
Mirrored Extensions is when you have one and the same extension line on several phones, in example, in the office, at home and on the cell phone. So when someone calls your extension all you phone sets sharing the same extension will ring.
AT&T Inc. is a premier communications holding company in the United States and worldwide. Operating globally under the AT&T brand, AT&T is recognized as the leading worldwide provider of IP-based communications services to businesses and the leading U.S.
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