Initial Actions COMMS

Whenever you do not receive a reply, the first thing to do is wait (for 10 seconds) and then try the call again. It could be that ATCATC —Air Traffic Control did not hear you because of a synchronous transmission, that they are speaking to colleagues, or some other reason. If after a second attempt you are still unable to raise ATCATC —Air Traffic Control, then Annex 10 Part 5.2.2.7.1.1 applies. The important sections are in bold.

When an aircraft station fails to establish contact with the appropriate aeronautical station on the designated channel, it shall attempt to establish contact on the previous channel used and, if not successful, on another channel appropriate to the route. If these attempts fail, the aircraft station shall attempt to establish communication with the appropriate aeronautical station, other aeronautical stations or other aircraft using all available means and advise the aeronautical station that contact on the assigned channel could not be established. In addition, an aircraft operating within a network shall monitor the appropriate VHFVHF —Very High Frequency channel for calls from nearby aircraft.

This can be summarised as: "Previous, Guard, Air to Air, Other (the four frequencies you can try)" and finally "other aircraft" (on original frequency.)