0870 numbers are classified as "Non-geographic" numbers.
They were formerly known as "national rate" numbers, but there is no longer a distinction between local and national rate numbers, so this description is now obsolete.
They are charged at the same rate as calling 01 and 02 numbers.
There is currently no revenue shared with the call recipient, but this may change, and they may be brought in line with 0871, 0872, and 0873 numbers, in which case the cost of calling 0870 numbers would increase significantly.
Many telecoms companies currently include calls to 0870 and 0845 numbers in bundled call allowances.
Calling 0870 numbers from a mobile phone can cost significantly more than from a landline - up to 41p per minute.
Here are some links to major telecoms companies, so you can check the tariff that applies to you:
0870 numbers can begin as follows:
The correct way to write telephone numbers beginning 0870 is in the format 0870 xxx xxxx