The subscription cost would have to be significantly lower than the cost of purchase, for me to be interested, due to the potential resale price of an iPhone. If I was cynical, ha ha, I would suggest that Apple would be hoping that a large number of users would not upgrade when they became eligible. Companies make good profits from people who continue with their old phone for years after the initial contract has finished. O2 contracts are part phone cost part airtime so that when the phone is paid for you only pay airtime. I opted for 3 as they were £6 per month more than 3.
