After deciding to switch my service to Airvoice (AT&T network) I registered here to gain some insight. Read about issues with AT&T not prorating final bills if cancelled in a billing cycle (per customer contract).
I suscribed to Airvoice 4 days after my billing cycle ended and expected AT&T to give me heartburn but was pleasantly suprised to recive a prorated final bill. Albeit the prorated bill included a $5.00 late fee (which was reversed after a phone call to customer service). Perhaps all due, in part, to Airvoice being AT&T network but none the less I'm a very satisfied former AT&T contract/month to month customer.