It looks like an issue with the proximity sensor. The screen doesn't switch off when you're talking and you're just turning the loudspeaker on with your cheek. That's pretty common issue, I've heard a lot of it. As for myself, I've got this issue just twice - both times when calling outside in the daytime.
I had the same a couple of weeks ago. On the support chat I was recommended to do a soft reset. So far I looks like it did the job.
-Turn off the phone
-Turn the phone on by pressing both the power and volume-up button at the same time and keep them pressed until the screen shows the 'android one' screen. Then release the buttons and let it startup.
Soft reset does not erase apps, settings and data, but clears some buffers and stuff.
Like I said this seems to be working on my phone.