Never ever used HF with the American controllers in Kabul always got through easily on VHF. Frequency is very busy most of the time and sometimes he (or she) but mostly he can be working aircraft you can't hear as well as the ubiquitous 'Shell' tankers out of Manas. Give him your estimate for his boundary entry and exit and if there's a heading change and subsequent level change required then be ready with a positive call of what level you want and don't dither.....like I said very busy and the super hot American controllers don't suffer fools gladly