Seems perfectly clear to me.
I have evening and wknd calls on my BT phone which means something like i can call for free after 6pm and before 6am and from friday 6pm till monday 6am.
If I begin a call at 5.50am during the week and dont finish til 6.30am then i`m liable to pay for 30 mins of the call. Its irrelevant what time I begin the call!
I`m certain no-ones attacking you but to be honest I dont know what the problem is!
You can go home early but you have to pay as discussed, seems to make perfect sense.