The short answer is, Nothing will happen!
Now you will think, then why flight attendants tell the passengers to either switch off the phone or put it in flight mode during takeoff and landing?
The reason is, during takeoff and landing the pilots are always in contact with the radar and control room. The were getting important guidelines about flight through headphones. And if mobile phones are not placed in flight mode then it may create interference with the signal they are receiving which will create a disturbance for them. In such if pilot fails to listen to an importance message from them then it can be a big chance for an accident.
What is flight mode?
Flight Mode or Airplane Mode is a setting for mobile phones and other portable devices that switch off all wireless activity – like Cellular Network, Wi-Fi, Bluetooth and GPS.
You are allowed to listen to music or watch videos or play games once the plane is taken off. This is to ensure the safety of the passenger.
So better follow the rules to ensure yours as well as passenger safety when you are boarding a plane!