This is a question I was asked recently and it illustrates just how confusing the current 08 number range is, covering everything from free calls to premium rate.
numbering plan, the 08 prefix was set aside for special rate calls, a tag that
can cover a multitude of sins. Back in the day, when your choice was a BT
landline or a BT landline, calls to 0800 numbers were free (with the number's
owner picking up the tab) and 0845 were designated as "Lo Call" and
were always charged at local call rates.
With the liberalisation of telecoms, many more operators started up and offered new services and tariffs, so that today we have a much more complicated picture. The standard for what you should be charged for a call is still the BT landline, but it can actually vary between operators, especially mobile operators and will also depend on what inclusive call packages you have.
A simplified view is:
080 numbers are free to the caller from a standard landline, but mobile operators, for example, may make a charge to cover what they perceive to be a reasonable cost for use the of the air channel. Vodafone sets this at 14 pence per minute. If an operator does levy a charge, they must include an "advice of charge" message before the call is routed and allow you to end the call before charging starts.
0843, 0844 numbers should be no more than 4.26p per minute or per call from a BT landline, but the charge made by other operators may be different.
0845 numbers should be charged at the BT local call rate, but the charge made by other operators may be different.
Surprisingly, perhaps, 0870 numbers, which have been the source of much controversy, should cost you no more than calling a normal geographic number (01, 02 prefix), and furthermore, if you have an inclusive call package, calls to 0870 numbers must count towards your inclusive call minutes.
0871,0872,0873 numbers are higher rate special services and will cost you up to 8.51p per minute from a BT landline and may be more from other operators.
If you have an inclusive call package, then except 0870 numbers, your calls to other 08 numbers are not likely to be included.
You may have heard that in June this year, a new regulation was introduced governing the use of higher rate numbers for customer support lines. If you need to contact your supplier in connection with a purchase you have made, you cannot be charged more than the normal geographic rate for your call, which has effectively ruled out most of the 08 number range for this purpose.
In June 2015 we will see a further development which is intended to make it clearer who is charging you what, with the connection service charges being separated out. This will be quite useful when you come to look at your bill, but it remains to be seen how much it helps you to anticipate before hand how much a call to a particular number will cost. The good news, though, is that as part of this change, calls to 080 numbers will become free to the caller wherever they call from, including mobiles.
See also Ofcom's page on telecoms numbering