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TUCoPS :: Phreaking General Information :: cards.txt

BT Fonecards

BT Fonecards

An introduction by NeonDreamer (18/5/96)

  The BT fonecard is a credit card sized sliver of plastic, coming in various
denominations - catering for the casual and heavy users of payfones.  Sold
from vending machines and newsagents they are considered to be a convenient
alternative to a pocketfull of change.  They are likely to decrease in
poularity with the introduction of credit card enabled payfones.

  The mechanism by which this system works is based on reflective
infrared readers and electrical discharge writers.  On the printed side of
the card there is a reflective magnetic strip, covered by a layer of white,
infra-red transparent ink.  This strip holds all the important information
related to card verification and remaining units are stored.

  The BT cards are very similar to NYNEX charge cards, which have the
validation information encoded as areas of high and low reflectivity in
the left hand two centimetres of the strip.  The remaining units is encoded
as an area of high reflectivity which fills essentially the rest of the

  When you insert the card into a payfone, the validation information is
scanned, and if validation fails the card is prompltly rejected.  This
is AFAIAC just as likely to be a problem with the payfone than the card,
especially in older units.

  When a call is made the fone fires a spark across the write head - which
has the rather crude effect of converting the area from high to low
reflectivity.  These can be observed on the card as small scorchmarks.  Value
is deducted when the call is completed, giving the user a visual idea of how
many units are remaining.  It is also understood that in the event of any
faliure to mark the card during billing the card is ejected and the call

  You will hear a million urban myths about ironing these devices, putting
them in microwaves etc to get them to give more than their units worth - all
of which are entire nonsense.  Unless you can patch up an already used card
to the reflectivity of old, you cannot tamper with the system - certainly any
measure to foil the physical marking of these cards is doomed to faliure.
It should be noted that most cardfones will allow you to make 0800 calls and
0500 calls (AFAIK) without inserting a card.  Thankyou BT!

This phile was adapted in part from "The NYNEX charge card" by Kevin Daniel
which appeared in the Spring 1994 issue of 2600 magazine.


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