im not sure i would use 0870 as they although can be charged national call rate, they can also be for profit like 090 but not to the same extent,
the goverment departments like DVLC had to drop 0870 for 0845, because they were critisied for making profit on callers on 0870,
i wont phone them, not sure the publics to savy on it anyway.
ie. your charged for the call plus a bit on top for them