084 and 087 numbers have always been expensive from mobiles.
03 numbers were introduced in 2007. These are usable in inclusive minutes from landlines and, crucually, from mobiles too.
03 numbers cost 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).
In 2013/2014, the Consumer Rights Directive will force many of the users of 084 and 087 numbers to move customer service and other similar functions to 03 numbers.
034 and 037 numbers are already reserved for the users of the matching 084 and 087 numbers.
New users can choose 033 numbers, and certain organisations can use 030 numbers.
All 03 numbers are charged at the same rate.
See also
http://en.wikipedia.org/wiki/Non-geographic_telephone_numbers_in_the_United_Kingdom#030.2C_033.2C_034_and_037_prefix
Until that happens, look for an alternative 01 or 02 number to ring in place of the 084 and 087 numbers.