What we (industry wide) need is some standard phone call that can be made. E.g. "I am not fit for duty". End of story, no questions asked.
We should not need to explain if it is a blocked sinus or an 18 month old baby screaming all night that has prevented sleep.
As it is, when sick, there is no requirement to disclose the nature of the illness, and this should be the same for fatigue. Should be illegal to question it whatsoever.
When your call about fatigue is genuine, you will not be bothered if it is questioned. Of course when this is abused it is a different story.