I had this problem too but i was able to fix it. At least temporarily. Just follow these simple steps and let us know if it works for you or not:
# Go to Settings > Apps & Notifications > Phone
# Clear the Phone cache and storage
# Go to PlayStore and search for the Google Phone app.
# Click on un-install. This will uninstall the updates for this app.
# Restart your phone
That’s it. I followed this procedure and my phone has been working fine ever since. Here’s a Guide that includes screenshots: https://www.helpmegeek.com/fix-for-google-pixel-freezing-calls/