Sensors are not working and screen goes black

Hi,

I own an Iphone 6, Grey colour a 16GB model.

I brought the phone 7 months ago and my phone is running on ios 9.2.1

Sometimes when i am on call the sensors on the phone stops working and the screen goes black. No buttons will work at that time and i can't disconnect the call. After 2 to 3 minutes it returns back to normal and i can use it.

And sometimes if i go to contacts or main menu while i am on a call, I can't go back to my call screen, I can see that i am on a call on the top and I can talk too.

The worst part of the problem is that it happens only sometimes. otherwise it is perfectly alright.

I thought it may be because of low memory but i have 5.1 GB free space available on the phone.

Can i claim the guarantee for my phone and even if i go to claim how will the apple guys understand my problem as it happens only sometimes?

Help