How do you check the cost of a BT call?

(For anyone reading this outside the United Kingdom, this doesn't really apply - it's only for UK telephone users on a British Telecom line!).

I recently had to make a call to an 0844 number to renew some insurance. I looked it up on SayNoTo0870 and obtained the equivalent national prefix number (0292 something), but wasn't sure which would be cheaper.
I found this tariff guide on the BT website, which mentions that 0845 and 0870 numbers are free for me at weekends. 0844 numbers are generally cheaper than 0870 numbers, so I thought that the 0844 number might be best.

Well, it turns out I was wrong, and I was charged 47 pence for a 10 minute call (grr!). I found this out not by waiting for my bill, but by using a technique I found on this page.

Basically, by entering a special code into the phone before the number you're calling, the call is made and you are immediately called back by the telephone exchange with the duration of the call and the cost.
The code is: *40*number#
(where number is the actual number you are calling - e.g. 084400000000).

This service appears to be free. There are a number of other "access codes" for BT exchanges listed on that page, most of which cost money. I guess there's a chance that even this service costs money, since the list is over 10 years old!