Calling a geographical number from a BT residential landline is certainly cheaper than calling a premium rate, disguised premiums rate, personal, mobile or international number.

Personally, I pay a total of 4p for a weekday daytime call of any duration to any UK 01/02 number and nothing for such a call at any other time (although, on the rare occasion that the OH stays chatting for more than 85 minutes in the latter case, I do have to 'remind' her to hang up and dial again to make sure I don't get a bill).