A very convenient way to display a message on an IP phone is to configure an idle URL. The IP phone does an HTTP GET to the specified URL (the phone screen is basically a browser). The file type hosted at the server may be XML, whith directives, tags, PHP scripts, etc.
There is also a way, more involved, in which XSI can be used to trigger asynchronous events on the phones, without the phones having to request it. There is an XML tag called <CiscoIPPhoneText> that could be used here. Cisco however does not offer scripts to do this. Customers have to implement their own code, format the text message, clear the screen, etc.
The easiest way to broadcast a message to phones, is by means of the “system message” command under "telephony-service". Whatever text is entered here, will automatically appear on the bottom line of all phones with appropriate displays without having to reboot them. The text here is limited to just a few characters though.
Documentation on Phone XSI Applications:
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/all_models/xsi/6_0/english/programming/guide/XSIbook.html
