[cisco-voip] DIDs / ELINs for use in CER ?

[Dark Fiber]

I am getting a bit confused about something regarding the use of DIDs in CER.

The documents basically state that you need to get some DID's from the
service provider for each ERL you plan to define, these DID's will be
used as ELINs to both identify the location and for the PSAP to return
the call if need be.

So a 911 call is placed, and once things are setup CER will basically
do or have CM do a translation of the users extension to one of these
DIDs you specify so that the service provider  will know what PSAP
that call needs to get routed to and so the PSAP will have a number to
call back on and such.

All sounds fine and dandy I guess.

However, my experience (which isn't much) has led me to think that
usually when dealing with  DID ranges from a service provider that on
911 calls the service provider does not usually pass the DID of the
phone to the PSAP but rather the billing number which is usually the
first number of the range I thought?

This isn't a problem as far as passing the location, but it would be a
problem for a return call from the PSAP.  CER would be looking for a
call coming into one number which would be mapped to the phone which
called 911, however if the billing number was passed to the PSAP thats
the number that would be called back.

Hopefully I am just blanking on something here and this is not the case.

Can anyone enlighten me on what I am missing here?  Thanks!