This type of request comes up every so often, and usually the need for total restriction is always tempered by the need for 911 - as this should be available at all times to anyone.  The indicated solution usually is to use the AXL SOAP API with your application to change the available calling search space on the device, so that the phone can only call 911.
 
You could also implement this so that the app used the Extension Mobility API to login a dummy E/M user, who had CSS for 911 only, had a special set of services that only pointed to the unlock app, etc.
 
Beyond the more or less standard admin config options - dial plan changes, E/M, line provisioning, services provisioning: available via AXL - there is no mechanism to take direct control of the physical buttons on the device.

Dear David,
                  plz let us know the details about how to lock an ip phone without using EM service,  which axl functions you have used for  ip phone lock application. Can we implement security code mechanism while the phone is idle for one minute, but how let us know the technical details of the required axl functions etc.
 
Thanks

Dear David,
If i call the xml service having the axl request of <>updatephone> as an idle url after setting the idle time of 60 seconds, is it possible

Thanks for ur suggestion

Dear David,
Would you let me know, how to implement phone locking with CME and can we use axl in order to do so,
Thanks