proximity sensor issue

That's a normal behavior. Proximity Sensor, when you put the iPhone on your ear, should make the screen black and unresponsive. When you take the phone away from your ear, the screen should revive.

Make sure that nothing (cover or screen protector) obstructs or interferes with that sensor.

Regards

Giulio

As already stated, if you're using a cover or screen protector try to remove them to avoid interferences.

If the issue persists, then yes, your proximity sensor might be defective.

I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.