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.
Citel PBXgateways are installed at the central PBX location, and EXTenders are deployed at branch offices, call centers, or at home offices over managed IP connections, DSL, cable, or analog circuits