Hi Jillfoalltrades,
It sounds like your screen is unexpectedly turning off while you are on a phone call. We can help with this!
If you haven't already, let's go ahead and restart your iPhone using these steps: Restart your iPhone
Also, be sure to complete any available software updates for your iPhone. You can find out more information about updating your iPhone here: Update your iPhone, iPad, or iPod touch
If the issue persists and you are not using a case or screen protector on your iPhone, it's possible there may be an issue with your iPhone hardware. Our service options vary by country/region and you can find out what service options are available in your location here: Find Locations
We hope this helps.
Take care!