E911 restricted telephones
You might have telephones from which you do not want users to make or receive external calls. You can put such telephones into TTI service. However, an improved method is to give the telephone an extension number, and then use a Class of Restriction (COR) to restrict the permissions.
For example, if someone dialed 911 from such a telephone and the call accidentally dropped, the public safety officer will call back to the calling party number that the public safety office received during the 911 call. If the telephone that dialed 911 has an extension number, the telephone receives the return call. If the telephone that dialed 911 was in TTI service, the return call rings someone else who does not know about the emergency.
