http://community.freepbx.org/t/intercom-door-opener-w-freepbx/21994/3
http://www.ebay.com/itm/IP-Sip-door-phone-intercom-opener-electric-bell-for-Asterisk-Avaya-Alcatel-MAUS-/121525848327
http://www.aliexpress.com/store/product/Security-SIP-door-phone-asterisk-compatible-PoE-model/500745_1547592378.html
http://niteray.com/
http://www.cyberdata.net/products/voip/digitalanalog/intercom_endpoints.html
http://www.2n.cz/en/products/intercom-systems/ip-intercoms
