Whether making or receiving calls the screen does not go blank. So my cheek keeps either muting the call or disconnecting it.
Back in the day this was an issue see if any of these helps or helps you identify the issue.
Long hair was an issue with the proximity/light sensor. Hair covering the ear would allow light seepage and the phone wouldn't recognize that it was being held up to the ear and the screen would stay active. These days however gyros and position sensors can also play a part. Try making a call and with the phone in an upright position cover the top of the screen and see if the display goes dark.
Another issue was makeup or dirty screens. If the sensor already thinks it's dark then the display may not dim because the phone doesn't realize that there's been a change. Clean the top of the screen and repeat the call test.
Finally it is possible that the sensor is just not working. You might be able to see this if you set the screen brightness to Auto and then shine a light at the upper portion of the screen. See if it brightens. Conversely cover the top of the screen and see if it dims. If both of those work report back and we'll see what else we can come up with.