Does the screen not work at all if you tap it, or is it just not waking after ending a call and moving the phone from your face? If the latter, it sounds like a proximity sensor issue. Have you cleaned your screen lately (use a microfiber cloth, slightly dampened). Do you have a case on your phone which could be blocking the sensor? Have you tried a Forced Restart on your phone --> Force restart iPhone - Apple Support
If nothing helps, it may be time to schedule an appointment at your Apple Store Genius Bar for evaluation and options --> Genius Bar Reservation and Apple Support Options - Apple