A short prefix which, when dialled before any number, tells you how much the call will cost, including any (evil but increasingly common) flat rate 'connection fee' and a per minute fee. You can then press a key to make the call, or hang up and write a letter / send an email instead.
I defy anyone to pass a test on what a series of numbers will cost to call without looking it up. There are fifty two A4 pages with three columns in smallish print of numbers and their
cost tariff codes – the four pages saying what any particular code actually costs is in another document!
07021 70.. is different (the PNS2 rate table) to 07021 71.. (PN18) and both are different to 07021 72.. etc down to 07021 76..! That's seven significant digits you have to know before you can know the cost! There are twenty one columns of 070 numbers alone.
Seven isn't even the record for significant digits: see 0844 0010 (I22) vs 0844 0011 (I23) vs 08440012 (I24) etc, where it's eight you need to know. The actual record is probably within 0845, where three numbers (eleven digits long) are free, and all the rest aren't.
It's not even easy to do a program to say what costs what. When it comes to mobile numbers, 07956 (FM3) is not the same as 07956 7 (MB1). Huh? Why isn't it covered by the former?
This is before you've got me going on why the basic 0845 tariff ('UK local') is the same as the 0870 one ('UK national') except that the latter is often free, depending on your package, whereas the former never is…
And Virgin Media are not the worst at this sort of thing! I still bear the scars of having to know some obscure details of the BT price list, while Cable and Wireless never could tell ex-work the basis for what they were charging us – I was glad when we were among the 90% of customers they told to find another carrier.