Okay, as utterly absurd as this request is. And we all agree on that.... I'm going to take a stab at it....
Some alarm systems will call in prior to the completion of the egress cycle to check for authorization by code. Others will call at the very end.
If it's one of the systems that dials in first:
Create a user shortcode for the alarm (the alarm has to be plugged into the IPO) that redirects it to a "post" and have it say something like "alarm system armed/disarmed". BTW, you can't have it count down as the alarm will get PO'd that it can't phone home within a certain period of time. It would have to wait for the "post" WAV to complete. Then forward the call to whomever is the monitoring company.
If you want to get fancy. You could toggle a variable that would correspond to the actual alarm armed/disarmed status.
Sorry to say, if it dials afterwards or they aren't monitored.... Well.. You're toast. Sort of....
Now, most alarm systems have an arming LED or arming relay output, you may be able to attach the arming relay to an analog phone port on the IPO and "shunt" the port off-hook. Put a "hot dial" to a VM shortcode and have it post the WAV. Have it disconnect at the end.
Lastly and probably most practical, tell them it's a phone system, not an alarm system. You can in many cases hook up "daisy-chained" keypads in other locations to alert the arming process.
Drew