From aa-asterisk.org.uk wiki
056 numbers are typically used for LIECS services and are charged by BT at the equivalent of geographic rate for most numbers. Officially they are g21 rate, category 11 in Section 2 Part 11 of the BT Price List. However there is no requirement for this and some operators (e.g. Orange) charge considerably more than this. Compare 03 numbers, which are capped at geographic rate for all operators.