As long as you have administered the system properly, you can always send the Emergency Location Identification Number instead of the actual station extension. Please see Chapter 71 of the Feature Description and Implementation document at:
https://support.avaya.com/css/P8/documents/100089836
for information on how Communication Manager supports 911 natively with no additional equipment and how to administer the various features. You might also want to look at Chapter 69 which describes how to allow calls from unnamed (unregistered) IP telephones.