Intercom

Intercom

Aiphone http://www.aiphone.com/home/
Commercial intercom systems can be as simple as a front door personnel verification tool for the front desk, to allow a receptionist to "buzz in" validated solicitors, or can be as versatile as a multi entrance, multi station video intercom for larger facilities.

Linear/M&S Systems http://www.linearcorp.com/communications.php
Residential intercom systems allow for multi-room music or link to the main TV with the added bonus of verifying front door entry from anywhere in the home. Calling the kids down for dinner is a snap with an intercom system, and station muting is standard on all indoor stations for private time.  Add an in-wall iPod dock or CD changer or use your existing audio equipment as an audio source to complete the installation. AM/FM radio comes standard on all units.

Teradon Systems http://teradon.com/
School intercom systems nowadays run on Cat 5e wiring and are much more reliable than older systems that used wire wrapping on the headend equipment.