In Infinite Flight, if someone doesn’t tune into your frequency you just tap on their aircraft on the map then Send On Guard Warning. Some may think this is a concept exclusive to IF, but this is not the case.
In the real world, all aircraft are tuned to a certain frequency at all times. This frequency is 121.5 MHz and is the international distress frequency. If a controller wishes to On-Guard an aircraft, they tune to this frequency and transmit the same message we hear in IF “ABC123, you’re in an active airspace, please contact Melbourne Center on 127.0”.
An example of this being used in real life is MH370. When the flight never contacted Ho Chi Minh Center on that fateful night, attempts were made to reach the pilots on the distress frequency.