Configure Asterisk paging and intercom.
goto
http://www.voip-info.org/wiki/view/Asterisk+Paging+and+Intercomthen configure a button on the phones or call the paging/intercom extension number from the phone.
For more info on Astra phones goto
http://www.voip-info.org/wiki/view/Aastrahttp://www.aastratelecom.com/cps/rde/xchg/SID-3D8CCB6A-B5DC9FA1/03/hs.xsl/27922.htmGood luck.