I know that you can have an hour free and put the phone down and ring again and get another free hour. I'm not sure what happens if you go over the time.
I'm obviously hoping that the first hour was 'free'
I just called BT to ask & the chap told me that if you're paying the surcharge for "Anytime Free Calls" (as I do) then you will be charged for the amount of time OVER the hour, NOT the 1st hour.
I'm with TalkTalk and regularly go over the hour with one particular friend but only get charged for the extra minutes. Not helpful I know but couldn't resist.