Phone calls to numbers like 0500, 0800, 0808, 0842, 0843, 0844, 0845, 0870, 0871, 0872 and 0873 can cost you a lot.
They are called non-geographical numbers. A lot of companies use these numbers, because it may be simpler for them to advertise or give customers only one number to call, but it's not always good for you to use them.
If you have an "all inclusive" or "flat rate" landline phone contract, it may include calls to all numbers in the country but only calls to geographical numbers, that is numbers starting with 01, 02, 03.
The same is true if you make calls from your mobile phone. Your contract most likely doesn't include calls to those numbers. For example, my mobile contract doesn't them and calls to 0800 numbers (which are supposed to be be free numbers, at least from a landline) from my mobile phone cost me £0.35 per minute.
Another reason many companies use these non-geographical numbers is that some of those numbers, especially 0870, make them actually earn money when you call them!
In some cases, like bank numbers, you can find an alternative number on the back of your credit or debit card. It would be listed as a number to call from abroad but this doen't mean you cannot call from your country. The number would be, for example, +44 1234567. You simply dial the number without the country code, that is 01234567.
If it is not on the card, you may search on the Internet for the company and you may find a geographical number. If not, when you call them on the number they give, you can ask them for a geographical number, tell them you need to call from abroad.
If you want to find the geographical number for most companies, I recommend a website I use all the time, www.saynoto0870.com.
There is a search box where you type the non geographical number you have or the company's name and you get one or more geographical numbers, with description of the different offices.
Many of these numebrs have been added by customers themselves.
There is also an app you can download to automatically search for alternative numbers.
They also have a page with links to other useful site to save money.