I would suggest any extensions that are or can be mobile, hard or soft, be redirected QUICKLY to a BRIEF but succinct message that the call must be made from the users cell phone, all the warm-ware units using such mobile extensions will have been previously well drilled of the need to NOT USE the softphone or hardphone for 911 calls.
A) Cell phone service will use GPS to locate the call for the E911 service they get connected to,
B) Every user of these mobile extension will either have a phone in their pocket or just use the ‘other button’ for such calls, most Android softphones allow direct integration with phone dialing , some allow controlling the ‘trunk’ used by dial pattern