word usage - 'Call up' - what does 'up' mean?

call up : to make a phone call to a person or a place

Call up and make a reservation for eight o'clock.
Can you call up for me and tell them I'm sick?

In these sentences, I don't know what does 'up' mean?

I (non native) understand what 'up' means in these sentences:

I looked up at the sky.
The flag is up.

But when we use 'call up'... What nuance of meaning of the word 'up' is here? Is it possible to omit 'up'? Would it still be the same meaning?

And these sentences do not have 'up':

I'll call again later.
I called the office to tell them I'd be late.

So what is difference between 'call' and 'call up'? What nuance does 'up' take?

Answer

Call someone up meaning to to "call someone on the phone" is an idiomatic expression. "Up" in this specific case is an intensifier, it is used to stress the action you are going to do:

up (adverb):

• 12) Used as an intensifier of the action of a verb: "typed up a list."

AHD