To send a message to a cisco IP phone you can use the CiscoIPPphone interface via http://phoneip/CGI/Execute and send the message via XML.
Figuring that out isn’t too difficult if you just google some examples.
However, authenticating can be a trick as can be finding out how to authenticate correctly to any phone (assuming you have the rights to ..eh.. give yourself those rights).
Phone “Device Association” is single user for end users.. so you don’t do this through your admin or cucm account. You need to setup an Application User in CUCM and then from there you can select any device for association without messing anything up for the user or his/her’s phone.
Maybe I’ll come back and put some screens/examples of the process.. but this is really just a note-to-self.