Phone numbers beginning with 08 and 09 can cost a lot more to call than a basic rate landline.
Some of these numbers are typically expensive to call, regardless of your phone operator, and are rarely covered by the 'inclusive' minutes you have as part of your mobile or landline deal.
Our table reveals the most costly phone numbers and how much you can expect to pay.
However, there are a number of tactics you can use to avoid 0870, 0845, 0871 and other expensive phone numbers.
Here are five steps to finding a cheaper number and reaching these phone lines without incurring a hefty bill.
Go further: 50 ways to save money – our all-inclusive money-saving guide
How to check for a cheaper phone number
1 Use a company's 'calling from overseas' number
The organisation you want to call will often have a 'calling from overseas' number that starts +441 or +442.
These will be standard geographic 01 or 02 numbers, which are likely to be cheaper to call. Just replace the +44 with a '0' to call the number from within the UK.
2 Try calling '03' instead of '08'
Following new regulations covering the use of non-geographic phone numbers, many companies have switched their customer-service phone line to a basic-rate geographic phone number.
The majority of these companies now use the exact same phone number beginning with '03' instead of '08'. However, some have struggled to update their marketing materials with their new phone number, so if you see an '08' phone number advertised, try dialling the same number beginning with '03' first.
3 Call the company's sales line
Companies may use cheaper – or even freephone – numbers for their sales lines to encourage prospective customers to call. If this is the case, try calling the cheaper department and ask to be transferred.
4 Call head office
If the company has a geographic number for its head office, try ringing that and ask to be put through to customer services.
5 Try saynoto0870.com
The website 'saynoto0870.com' lets you look up the company you want to call or the 'official' 084 or 087 contact phone number it provides. It then lists cheaper or freephone numbers for the company.
Switch your home phone or mobile provider
Charges to call non-standard phone numbers vary by phone operator. Check their published tariffs, taking care to note the access charge for calling 084, 087, 09 and 118 numbers.
Our guides on how to choose a phone bundle and how to choose a home phone service show how much you could save.
How to dispute unfair phone call charges
Get your money back
If you have to call an expensive phone number to make a complaint – perhaps because a product or service has a fault or the company concerned hasn't fulfilled its side of a contract – ask the company to refund the cost of any calls you've made.
Contact your phone service provider too, and ask it to suspend the disputed part of your bill until you can sort out the problem.
Premium rate phone number problems
Phone-paid Services Authority (PSA) is the regulator for premium rate services (numbers beginning 070, 0870, 0871, 0872, 0873, 09 and 118). It handles complaints about the lack of call-cost information, misleading claims, offensive content and prolonged calls. It is also the watchdog for premium rate text messages.
To contact the company, call 0300 303 0020 or visit its website.
Phone billing problems
If you think your phone service provider has charged you in error, complain to it. If it won't help, contact Ombudsman Services or the Communications and Internet Services Adjudication Scheme (Cisas). All phone companies must belong to one of these two alternative dispute resolution schemes – although third-party retailers of phone services and handsets aren't covered.
You can only contact Ombudsman Services or Cisas if you've had no success with a formal complaint to your phone company and eight weeks have passed, or if the phone company has said it won't do anything else.