Alternately you could use the ARS Maximum Dialed Digits form to keep if from dialing anything.
Assign the phones a unique COR such as 60 then in the Maximum Dialed Digits form set the number of digits allowed to be 0 for that COR.
Then the device can hear pages and take calls but will be unable to dial anything.
Then to keep it from anyone on the outside being able to call it, change the extension number to a non-DID number.
That works if you're only trying to block outside calls coming in.
Ralph