As bpere says above
056 numbers are supposed to be charged at standard landline rates for the calling party in the U.K. (I say "supposed to" as this is the way it should work in theory, but it depends on the provider).
I have just obtained an 056 number from:
uknumber.co.uk - Giving you a free UK phone number - wherever you are in the world!
Tell all your friends and family to use a BT line (domestic /residential /business NOT a payphone)when using an 056 prefixed number and then they will get "standard landline rates" which are 3.25pence for up to 60 min plus 6p call setup charge
You can check with BT's online price list here
BT Price List