iPhone 5 goes to sleep

[Iesa13]

I have the most annoying issue with my iPhone 5 since new, it randomly goes to sleep, when in standby mode I.e. screen blank, no matter which button is pressed or how many times the phone does not wake up, also when it's in this state even if you send a SMS, what's app or call the phone it still doesn't wake up. This can happen several times in one day then not at all for a few days, and the periods the phone is asleep for are not uniform, sometimes 3 or 4 seconds, sometimes up to 30 seconds and longer. I live in the UAE and took the phone on contract from Etisalat the telecom provider here, but when I complain to them about the issue they want to see it, but for sure because the problem is intermittent by nature it never happens when you want it to. I have googled this but haven't found anything the same, some similar about unresponsive home buttons, but none where every button is unresponsive. Anybody else facing the same issue, and if so any ideas on how to solve this - the obvious one of replacing the handset is proving difficult here ?