Why can't you use a Voice Gateway for Anveo 911?

[restamp]

In another thread, it was pointed out that in order to use RingTo, one has to also dedicate an SP trunk for Anveo 911 service.  I've never understood why this is necessary.  It appears to me that since 911 can only originate calls, a Voice Gateway could be used to provide 911 service.  I'm sure I'm missing some salient point, but could someone explain why this is not possible?  Thanks.

[N7AS]

With any 911 service, the 911 operators may need to return a call if needed.

[SteveInWA]

With any 911 service, the 911 operators may need to return a call if needed.

What Grant said.   

Consider this scenario:  you're having a heart attack or stroke, or some other major emergency, or you are attending to someone who is having such an event.  You make the 911 call, talk briefly, and then get disconnected, or you have to hang up to deal with the situation, or you temporarily lose consciousness, etc.  The dispatcher or other emergency responder can't call back.   Don't even bother setting up E911 service if it isn't going to provide maximum benefit when needed.

[restamp]

So instead of telling Anveo what DID to associate with a 911 call, they assign their own DID which will realistically never be used?  Seems a waste of increasingly scarce NANP numbers, but I guess if this is the case, it would preclude the  use of a Voice Gateway.

[SteveInWA]

There are a couple of reasons why it's done this way.  For example, E911 service is tied to a street address.  You may have multiple DIDs, or you may be spoofing a DID that is actually somewhere else, or you may be making an outbound call on a SIP provider without even having your own inbound DID.  In these cases, giving the E911 PSAP one guaranteed/known telephone number is the way to ensure that the number matches the correct street address.