I pay $176/month for 3 lines with AT&T and all 3 lines still have the unlimited data @$30/month on them. I can buy 3 subsidized iphone 6s for $600+tax total ($2

I pay $176/month for 3 lines with AT&T and all 3 lines still have the unlimited data @$30/month on them. I can buy 3 subsidized iphone 6s for $600+tax total ($200 each) or I can buy them each at $650. The price per month works out to about $27/month per phone. Will AT&T knock $27 off per month per phone if I buy them outright? The answer is no. I've talked to supervisor after supervisor about it and there is no discount if I upgrade and buy the phone outright up front. So I ask, where's the incentive? AT&T is by no means perfect but given all the traveling I've done and my experiences with Verizon and T-Mobile (never tried Sprint) I've found that AT&T and Verizon are interchangeable and T-Mobile is not quite on the same level. I know my rate plan hasn't become more expensive when I upgrade so how can I expect that it will become cheaper if I bring my own device?