[Sipping-emergency] Call hold

 From some of my notes (sorry, original source lost). In any event, 
doesn't contradict the earlier discussion, just probably indicates that 
this is a requirement we can punt on for now. (In any event, I don't see 
how this can be anything but an end-system feature. Knowing that a call 
is an emergency call is crucial to implementing any such end system 
services, so that's why the requirements document calls this out.)

Reverse reachability is much more important, in my opinion.

The full document (ANSI T1.628-2001) states:

-------------------------------
4.1.4   E9-1-1 Call Hold

E9-1-1 Call hold is an optional network feature provided to a PSAP which
prevents a caller from disconnecting an ESC.  Capabilities needed for
supporting E9-1-1 Call Hold are described in Clauses 4 and 5.  However,
there is no DSS1 or SS7 support for this capability at this time.
-------------------------------

Both documents clearly state that CALL HOLD can not be universally
supported, in fact it is clearly stated that a network reconnect - i.e.
call back will be attempted only - this is because the network may not
have the actual callers station ID, but some other number, reflecting
maybe the billing number or a location based "equivalent number" in the
case of larger organizations.  The fact that SS7 does not support this
provision will effect the ability of call hold via Tandem Offices as is
the case with most E911 offerings.

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