In my view, the only legit use of 084 and 087 numbers is for value added services, such as cheap international calls as mentioned in a couple of posts. They are really premium rate numbers with a low rate. Fortuantely recent legislation has barred their use for customer service lines, even if there are still some companies dragging their heels.Soon there will be a requirement for a breakdown of costs of such numbers to be shown to callers. Most of the money goes to the phone company, not the company you are phoning, expecially when calling from a mobile.
0800 and 0808 numbers will soon be free from all mobiles too. So free means free.
Call me an optimist, but with a bit of luck 084/087 numbers will soon be in decline. Ofcom also need to stamp out 070 numbers which look like mobiles but are another sort of premium rate used mainly for scamming.
i'm a simple soul who believes that any payment to the called party equals premium rate and belongs in the 09 range.
Until we reach phone number nirvana make good use of:
http://www.saynoto0870.com/ to find alternatives to 084 and 087 numbers.