Step 1: Buy a top up voucher of your choice, find a friend who is also on the same network to buy one too.
Step 2: Both of you type in the digit of the top up card into your phone (the same voucher).
Step 3: When 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.
Step 4: Repeat 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!