What is wrong with the 0800 number which does not cost?

If you are calling from a mobile then use the dial-through geographical numbers for use from mobiles are 020 0222 0900 and 020 0222 0700. Using one of these will allow calls to 0800/0808/0500 so-called "freephone" numbers. Charges will be as geographical calls, thus will often reduce the cost from mobiles.

. Dial one of these numbers and press the green phone off the hook button (or its equivalent on your phone). Once it answers, it will ask you to input the destination number. Enter the 0800/0808/0500 number but do not press the green off hook button again (as that will initiate a second call to the 0800 number which is what you're trying to avoid).