Do you have the same issue if you follow the OP’s steps to reproduce it?
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This happens to me on just about every flight. ATC starts normally and then is grayed out. Sometimes if I change the ATC frequency in the aircraft radio box the ATC screen window will tell me to contact the appropriate agency and I can start again. The other day I lost ATC and discovered that the automatic selection of the correct frequency in the ATC window was not changing the frequency in the aircraft radio box. I then had to manually apply every subsequent frequency change, and this worked until I was on final and then the tower audio stopped working as well. The ATC system has become far worse since Sim update 10, which coincided with my adoption of a non Microsoft AI traffic system, initially AIG, but now FSLTL. I wonder if the system is becoming overloaded with calls from an increasing number of AI aircraft. In addition it now seems that there are longer pauses between selecting a response in the ATC window and the audio system reading the reply. If the system could be dissuaded from making unnecessary traffic information calls on aircraft that are vertically separated, or passing well clear, then it might be able to provide a reliable system.