Often companies have other numbers than 0870s that will enable you to reach the same route. The website SayNoTo0870.com has a searchable alternative number' database to help you work around the charges.
Most big companies have a range of numbers. 0870s are commonly for complaints and enquiries, while sales lines are cheaper or even free (ie putting complaints off and encouraging sales). So dial the sales number and ask to be put through to the relevant department.