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From: JSOL%BUCS20%bostonu.CSNET@CSNET-RELAY.ARPA (Jon Solomon)
Newsgroups: mod.telecom
Subject: Rochester telephone service
Message-ID: <[BUCS20].JSOL.27-Oct-85.18:36:42>
Date: Sun, 27-Oct-85 18:36:00 EST
Article-I.D.: <[BUCS20].JSOL.27-Oct-85.18:36:42>
Posted: Sun Oct 27 18:36:00 1985
Date-Received: Sun, 3-Nov-85 03:41:18 EST
References: 
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Approved: telecom@ucbvax.berkeley.edu

Well, basically it is a 3-fold process:

1) your local exchange gobbles up as much of the number as can be
processed locally: In Step exchanges this is usually a "1", Crossbar
and Electronic exchanges usually gobble the whole number and do their
own translation/verification of the prefix (at least for calls within
the area code), then it is passed to (2).

2) The local exchange sends the call to a toll switch. Step in this
case passes the call as soon as the "1" is dialed. An interface to
the toll switch gobbles the rest of the number up and passes it to
the toll center using MF tones. The toll center dials a call to
the area code chosen, using both LOC and Common Carrier lines
(i.e. AT&T, Sprint, MCI, etc), connecting it to (3).

3) The local switch in Rochester then finally gets the call and
dials the local number. When the call is answered, supervision
gets back to the originating CO to begin billing.

"Route Planning" happens usually at the Common Carrier level.
At this point, at least, there is no mechanism to do route-planning
at the local switch (plans were made at one point to implement that
on a local switch, but I think divestiture put a monkeywrench in
the plans).

Therefore using this model, it is safe to say that your tones don't
do much in the process of completing the call. Special tones (not
dialable from a touch tone phone -- security in obscurity!) are
used to complete calls (except for step by step which dial pulses 
into the exchange).

Hope that helps.
--JSol