Many organisations who list an 0870# will have listed an International # for overseas clients/customers as the non geographic prefixes do not function from non UK exchanges. Examine(for example) the number provided on a bank/credit card for use from overseas in the event of emergency. Ignore the international dial code prefix, treat it as a normal UK # and guess what ? You are connected, and more often than not to a person rather than a machine, and far more quickly. In addition you are saved the ghastly and mind numbing experience of being treated as a victim by cynical money grabbing organisations that demonstrate the esteem in which it holds it's customers by fobbing them off, treating them like sheep and  doing this by using a machine.

It might just show them the area code you called from, not the full number.