Also, not previously mentioned, when using HF, you'r not speaking with a real ATC specialist, bUt rather a non controlling specialist who relays position reports and requests to the ARTCC which is responsible for aircraft separation.
Position reports must be in a standard format and as long as you receive a good SELCAL check, you don't have to continueously monitor the frequency. Someone with more technical knowledge can explain how the SELCAL check works.
Cheers, D.L.