Verizon: "Cancellations will become effective on the last day of that month's billing cycle, and you are responsible for all charges incurred until then." (http://www.verizonwireless.com/b2c/globalText?textName=CUSTOMER_AGREEMENT&jspName=footer/customerAgr...
What Happens if My Postpay Service Is Canceled Before the End of My Contract Term?)
T-Mobile: "If you port your number, you will be responsible for all usage and Charges through the end of your current billing cycle." (http://www.t-mobile.com/ - terms and conditions link at the bottom of the page, 3. * Your Term of Service and Early Termination Fees)
Sprint: " If Services are terminated before the end of your invoicing cycle, we won't prorate charges to the date of termination and you won't receive a credit or refund for any unused Services." (https://shop2.sprint.com/en/legal/os_general_terms_conditions_popup.shtml#3 - Your Right To Terminate Services)
I could not find mention of AT&T's terms regarding whether it prorates or not.