If you fly through Turkish airspace to Erçan, as destination, I would say, to only deal with Erçan, frequency 126.7 as you would anyway start your descent around MUT VOR, to cross DOREN FIR at a somewhat low level, you really do not have to deal with Nicosia.
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Your alternates should be either Adana or Antalya. Do not make it Larnaca...!
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If you come from anywhere in European/Mediterranean area, through Athens and Nicosia FIR, do not put Erçan as your destination, file it as Antalya (maybe) then upon entering the Ankara FIR, change your destination. Turkish ATC will immediately understand
why you did so, and amend your route/destination clearance. I was based in Erçan, on ACMI with KTHY in the 1970s with a B-720, and that is the way I would have done it back then.
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When you cross Turkey FIR from MUT through VESAR towards NIKAS and Banias, you have to work with both VHF frequencies Erçan and Nicosia. Nicosia ATC is then your controlling agency, your transponder code is the one Nicosia gives you, just inform Erçan (as courtesy) of the Nicosia clearance. And I am sure they "monitor" what Nicosia tells you.
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Happy contrails