There are no special trunks to the PSAP from a PBX.
What Sean appears to be saying is he recommendeds getting a POTS line
as a backup to a ds1 for 911, but it's a tad-bit more complicated than
that and involves equipment types and tarriffs of CLECs vs. ILECs in
router determination.
I agree with him on that mostly, but there's more to think about. SMB's
are not that sophisticated, are cost sensitive, and typically don't
have the equipment to handle that as well, or, the expertise.
YMMV.
-M<