Why doesn't my screen go black on iphone 4?

While on a phone call - My phone stays lit up and it ALWAYS tries to facetime people or go on speaker… My fiances iphone 4's screen goes black and he doesn't have that problem. Is there a way to fix that

Hi

There could be two reasons. Both related to proximity sensors.

1. Your Proximity sensor is not working: When you keep your phone near your face, the proximity sensors recognizes your action and automatically activates the screen lock while on call.
2. A screen guard that you are using is blocking the proximity sensors? Please check.

If the problem still stays, the sensor could be defective, please take it to the customer care!