084 and 087 numbers (with the exception of 0870 from 2009 onwards) are revenue share numbers.
084 and 087 numbers are expensive from mobile phones. Mobile operators usually charge more than 20 p per minute for these calls. They can charge up to 45 p per minute.
Whenever you see an 084 or 087 number advertised, replace the 08 with 03 (keeping the rest of the digits the same as before) and see what you get. It will either be answered by the correct company or will lead to a "number not yet in service" error message.
03 numbers were introduced in 2007. These are usable in inclusive minutes from landlines and, crucially, from mobiles too.
03 numbers cost exactly the same as 01 and 02 numbers if you have to pay for them (i.e. from pay as you go mobiles and from landlines without inclusive minutes).
034 and 037 numbers have been reserved for the users of the matching 084 and 087 numbers since 2007.
New users can choose 033 numbers, and certain organisations can use 030 numbers.
In 2013/2014, the Consumer Rights Directive will force many users of 084 and 087 numbers to move customer service and other similar functions to 03 numbers.
If the 03 number doesn't work then
http://www.saynoto0870.com/ might list an 01 or 02 number that you can ring instead.
Recognising that some 08 numbers cannot be called from abroad, a number of companies advertise an alternative UK 01 or 02 number to call. Use this number or an 03 number whenever you can.
03 numbers are detailed here:
http://en.wikipedia.org/wiki/Non-geographic_telephone_numbers_in_the_United_Kingdom#030.2C_033.2C_034_and_037_prefix