goldstein@delni.DEC.COM.UUCP (04/10/87)
Bob Clements' observation about Boston's mass calling number is
interesting, since the prefix code 931 (mass calling) mapped into
261, but doesn't any more. About five years ago, NE Telephone _deleted_
617-261 from the list of active codes. Anybody who had such a number
presumably had it changed (I don't know of too many, though; probably
a couple of Centrex systems). Now it makes more sense...
re: "CLASS" services that offer calling number id: I don't think being
on a crossbar will protect your number. Crossbars have automatic
number ID on them, for billing purposes, and probably forward it to
the toll switch. Certainly steppers do! You only need an ESS to
be on the receiving end of such services.
There is no one standard technique for delivering calling number ID.
Some systems use separate lines. I'm told that the Bell of PA trial
uses 202-modem tones on the line between the first and second rings.
Primitive but it works. Of course this is not available in many
places yet, and probably will never standardize pre-ISDN. The ISDN
protocol (Q.931) for call control is basically symmetrical for
incoming and outgoing calls. When the network sends a call to your
phone, it may contain a calling number field, if you pay for the
service. Field trials are beginning...
fred