Faulty hang-up connections. Even when the phone 'looks' hung-up, it isn't. Fix the hang-up buttons so that they work properly, or replace the faulty phone. You

Faulty hang-up connections. Even when the phone 'looks' hung-up, it isn't. Fix the hang-up buttons so that they work properly, or replace the faulty phone. You'll very likely find that when you listen to the phone, and work the hang-up buttons with your fingers, that the phone doesn't actually hang-up.