ccb123, that's something which would cause me to seek out some help too. We'll take a look at this together, to make sure you can multi-task while on a call. The first thing we'll want to check is your screen display settings. To do that, go to Settings and then select Display. In Display, you'll check out the Advance option on the bottom. Once you've brought up the Advance options, select Sleep. This determines how quickly your screen goes black. What do you currently have set?
Additionally, it's always a good idea to do a simple power reset of your Pixel. This helps the phone clear its head. Have you done any power downs since this issue had started? Finally, are you using the standard Phone app on the Pixel, or is this occurring with a different app?