There are multiple ways to replenish your Pay As You Go account balance:

Purchasing a Top-up scratchcard

You can then top up your account dial the shortcode *101* Top up scratchcard number# and press send. Your new balance and expiry date will appear on your screen. This facility allows you to check the 14 digit number before sending. Alternatively you can dial Freephone 888 from your Pay As You Go number and follow the voice prompts.