You can call freephone numbers in UK, USA, France, Poland and Taiwan completely free of charge through Skype. No credit is required on your Skype account.
This will be useful for most people using a mobile if you have/download the app as UK 0800 numbers can be expensive through your mobile phone network provider.
It has helped me to contact a company whom I work with regularly over in the USA. And it's completely free - all you need is the internet.
I know it might not be of much help for many people, but it might really help somebody.
Direct link to info page:https://support.skype.com/en/faq/FA24/how-do-i-dial-toll-free-numbers-for-example-1-800-1-866-and-1-877-number-series
These are the numbers you can call FREE from the UK with Skype
France: +33 800, +33 805 and +33 809 numbers
Poland: +48 800 numbers.
UK's 0500 numbers, 0800 numbers and 0808 numbers.
USA's +1 800, +1 866, +1 877 and +1 888 numbers.
Taiwan's +886 80 numbers.