I’ve known about this great little web site for ages, but I haven’t used it that much until recently. It’s a great wee tool. When you call a company on an 0870 number it will not be included in any fixed-price-for-all-calls deal on your landline or your mobile. So, before you make the call, visit SayNoTo0870 (new window) and enter either the phone no or company name, hit search, and it will search for an alternative, 01 or 02 number. All of this also goes for 0845 numbers.
It only works for companies and numbers which have been put into the system, so if you find a non-0870 or non-0845 number for a company, you can enter it on the site.