dial-back security

[lauren at rand-unix.ARPA]

Using call forwarding to try force incoming call bypass probably
isn't a very good idea.  I can think of at least three reasons:

1) If the forwarded call is answered, the company will be paying
   for the call (however short it may be) since most businesses pay
   for all local calls these days.

   Also, it seems rather wasteful to pay the extra money each month
   for call forwarding on every incoming modem line, even assuming
   it is available.

2) Various resets at the telco central offices could cause call
   forwarding to be cleared spontaneously at any time.  It could
   take substantial work to always verify that call forwarding
   is still in effect.

3) Call forwarding normally results in a single short ring being
   applied to the primary destination phone as the forwarding takes
   place.  This single ring could well be enough to trigger the
   incoming modem and cause possibly undesirable effects.

The security problem with dial-back schemes is real, but like all
security systems this method represents only an attempt at perfection, 
never actually reaching such a level.  PBX systems with dedicated incoming
and outgoing trunks can avoid the problems entirely.  In virtually
all cases, however, it would seem that dial-back systems are 
certainly superior (in terms of security) to straight-dial modems.
Of course, they are usually also a much bigger pain to use for the
authorized callers, and may not be needed in most situations.

--Lauren--