check xperia z2 used or new topic

Hello guys, i need a quick answer on the following question, cuz i have the opportunity of buying a "new" z2 in a couple of hours, but i need some final advice.
A guy at my workplace, owned a z2. On the screen's phone, appeared a yellow stain so he sent it to warranty. They replaced his phone (he showed me the warranty in which orange completed "replaced", the old phone's imei number along with the new phone's imei number, the old phone's box and the new one's box etc). I checked in the service menu the total call time, which was about 20 minutes, so it's ok from yesterday when he received the new replaced phone to talk 20 minutes. Also the phone has no scratch on it.
But now it's my question? besides the waranty paper and the boxes and so on, i rely on the total call time to check if the phone is new or not. Now, in case of aa hard reset that counter in the se4rvice menu, resets also? or it remains the same? (in my xperia Z, reinstalling the software using sony pc companion didn't affect total call time). So any advices? Besides that can i look at other things to see if indeed the phone is new? Thank you