how a post paid bill will reflect if I port one number out/cancel on 2nd day of bill cycle ?

I have 2 lines on UN-limited plan plan and bill cycle starts on 30th every month. now if I cancel one line on 2nd day of bill cycle while bill is already generated for the billing period, would I be paying for both lines FULL for the bill period? If I pay in full for both lines will they refund some amount as the 2nd line was active only for few days during bill cycle?

Kindly suggest how to sort this before I cancel the line