Changed
I too have had this proximity sensor problem.
First off contacted Apple here in the UK who told me that hadn't come across this problem before.......😱
After two days of talking to them, restoring my phone to factory settings and blah blah blah, they finally agreed to look at the phone!
This was there offer: go to the local apple store see the clever bloke whos paid lots of money, i declined as the nearest apple store is some 60 miles away. There next option was, we will send you a box, post it back to us, we will take a look and "if" it is broke, we will send you a new phone.
Hmm, so the phone less that 2 weeks old, no offer of replacement or loan (without a charge) and we will see...
Finally, I went back into O2, fortunately just as they had had a delivery, who very kindly exchanged the phone on the spot.
New phone seems spot on and in fact seems a touch better.
I'm not sure if the model numbers are different for the two devices, so will take a look and see if I can see anything major in that.
Cheers😀