josh_marchant,
Most likely your feature codes have been changed from an 8 to a 9, but you can verify by looking in the feature codes section.
Go to System > Device and Feature Codes > Feature Codes > Clear System Alarm
The other thing you have to remember is that the phone needs to be an Administrator phone, but if you are seeing the alarm on the phone it is an Administrator phone. The last thing is they may have the System Flag SPCL Key Required For Feature Code Entry turned on which means you will need to press the SPCL Key prior to entering the code.
Also you could just clear the alarm from the front of the MiVO-250 panel as well. Press the circle button, down arrow, circle button, then press the down arrow until you get to clear alarm and then press the circle button again and then you are done. Try not to do Clear All Alarms as there may be several alarms and you will want to see each one prior to clearing it.
Thanks,
TE