1. Create a new line on a Cisco phone, and put the configuration into sip.conf as you normally would(go into 'Settings: Call Preferences: Auto Answer (intercom)' and then make the line you've just created as 'auto-answer'.

2. Here are the contents of /var/lib/asterisk/agi-bin/callall:

#!/bin/sh

cp /var/lib/asterisk/agi-bin/*conf /var/spool/asterisk/outgoing

3. Make sure to make the script executable. And then for every extension you have as an auto-answer, have a file like this

in /var/lib/asterisk/agi-bin:

Channel: SIP/2006

Context: add-to-conference

WaitTime: 2

Extension: start

Priority: 1

CallerID: Office Pager <5555>

So, for example, if you have three lines that are configured for automatic answering - SIP/2006, SIP/2007, SIP/2008,  you should have three files named 2006-conf, 2007-conf, 2008-conf in /var/lib/asterisk/agi-bin that get copied into the outgoing call spool directory every time you call extension 5555.

4. Now, dial 5555 from any phone and you should have one-way paging.

People who use the pager may have to get used to waiting 1-2 seconds before speaking to allow all the phones to catch up with the audio stream. All of the phones hang up after 20 seconds, regardless of if the person originating the page has stopped talking. Change the AbsoluteTimeout values to increase this interval.

If you want a really confusing loud mess, then change the "dmq" options to "dq" and you'll get an N-way conversation going with everyone who has a phone. Bad.

If you want a really interesting office surveillance tool, change the "dmq" to "dt" and you'll suddenly be listening to all of the extensions in the office, like some kind of mega-snoop tool. Useful for after-hours listening throughout the entire office.

NOTE: Paging tested with Firmware version 6.1