Here's how it works

The 911 call does not come from the same DID. For example, suppose the customer has a UK DID, then he has simultaneous calling, do the emergency services rush over to the UK? No.

So the customer has 1 US DID, that rings in two or three locations. Each location has its own VOIP number which is in turn registered for 911 services and calls to 911 from that number do NOT send out the DID unless of course it is from the line where that DID is registered to.

So you have VOIP#1 (primary which has a DID pointing to it) You then have VOIP#2 and VOIP#3 which are configured to ring simultaneously.

As I said yesterday, my provider does provide this service.