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Consider the situation when someone is calling you, and you are in a busy street. Is sending a message - I am outside correct English?
Although it does convey the meaning that I am not at home.
It is better to say 'I am out'. (Out- away from home etc.)
I am out. (I am not at home.)
'I am outside' means I am not inside the building. 'Outside' means 'not in a building (but near it)'
I am outside (of) London. (somewhere else but not in London)
'Out' can be used in place of 'outside'.
"I am outside" is not wrong altough it doesn't give any specific information where you are. It only says you are not home or under a roof in general.