What does it mean by "call deep on one's courage"?

They are two ways of describing the same thing; so yes, they are essentially the same.

I would use "deep". But "low" can also refer to the register of a person's voice. Like on a piano, the low keys sound the deepest, have the lowest pitch and a bass register, just like a person with a deep voice.

I think "deep" sounds better because it is more specific. Unless you specify "low in pitch" or "low in register", the word "low" by itself doesn't specify what property of the person's voice you're actually referring to, so I would stick with "deep" or "low-pitched".

Not a silly question at all. Bit of a long-winded response but yes, they are both the same :)

It means that for someone who is afraid of the dark (to give only one example) to then force themselves to enter a dark place, means they have 'Called deep on their courage." The level of courage is greater than that usually needed.

It means putting aside your fear and doing something you're afraid of.

It could involve running across a battlefield to save a friend. It could just be speaking in front of a group or standing up to a bully.

its means roughly the same thing as when a drill sargent suggests you bring it up from here, while grabbing his balls