But how do mobile phone vouchers really work?

[MJDale]

Yes I've read this weeks article on mobile phone vouchers which, although very informative, didn't actually tell me how the numbers work. I know that you scratch a panel off of the voucher to reveal a long number which you then put into your phone. And I understand that the phone - or rather the phone network - will recognise this number and then credit the phone with the appropriate amount of credit BUT what is there to stop me from sitting down with my mobile phone and just punching in random numbers until I am lucky enough to hit a valid one which credits my phone. Yes I know that several sections of the Theft Act 1968 stop me from doing this BUT is there any physical thing or device in the system that the phone companies have put in place to prevent themselves being defrauded?

[davidm]

I expect you could, in theory, punch in a random number and hit one which credits your phone. However, given the length of the numbers, this is probably so incredibly unlikely that the phone companies don't worry too much about the risk!

[satisam]

I remember reading somewhere that when a shop owner opens a new box of phone vouchers they have to call the phone company and quote a code on the box, this code then activates the 100 or so vouchers inside. (It could be the seed number for the algorithm which generates the numbers on the vouchers so they don't look sequential) This is why you can't just guess the numbers, there are only so many thousand active at any time and the numbers are so long, it is (effectively) impossible to guess one. Also once you have typed an incorrect number a few times your account is flagged and an operator will call you to find out what the "problem" is (you can try that for yourself...).