Buy a top up voucher of your choice, find a friend who is also on the same network to buy one too.
Both of you type in the digit of the top up card into your phone (the same voucher).
you get to the last digit both of you press confirm at the exact same
time so the same voucher registers on both of your phones.
this with the other card. You will now have twice the amount of credit
on your phone for the price of half. This works 99% of the time!