[VoiceOps] Blocking 911 calls

Yes, I know, "you can't," but there have to be exceptions. I just can't find clear answers. We have some companies with locations outside the US, and they will occasionally dial 911 by accident. Is there any reason we can't block the accounts outside the US from dialing 911? Of course, there's the possibility that someone brings a phone here or uses a softphone account on a US phone. That "shouldn't" happen. What do others do for users outside the US? If it matters, the users are in Mexico and Philippines, mostly, occasionally in other countries.