why has my iphone 4s warranty suddenly expired??

Hi

Has anyone experienced their warranty ending early??

I purchased an iphone 4s from a friend in september, he told me it had around over 11 months warranty, i checked this by using apple's online serial number warranty checker and it stated the iphone warranty would expire 13th September 2013.

I used the iphone with my tmobile sim for around a month. I then decided to buy an iphone 5 and sell the iphone 4s, i needed to know whether the iphone was locked to tmobile, i put an o2 sim in which worked, then when i put my tmobile sim card back in the iphone it wouldn't work. My friend who i brought the iphone from contacted his network O2 to get the iphone unlocked for me, it was unlocked by O2 a couple of days later and the iphone now works on all uk networks.

Before i went to sell the iphone 4s i thought i would double check the warranty expiry date and it said expired. I wondered whether it was because o2 had unlocked the iphone but i contacted o2 and they said their unlocking definitely does not void apple's warranty.

I have tried to get proof of purchase for the iphone 4s but o2 have only sent through an email stating when my friend started to used the iphone on o2.

Why would the warranty suddenly expire?

Regards