An idiom is an expression whose meaning is not readily apparent based on the individual words in the expression. This forum is dedicated to discussing idioms and other sayings.
It depends on what you want to say. When it is a statement:
Me llama todo el tiempo. -> He/she calls me all the time.
Me llama todo el día. -> He/she's calling me all day long.
In the case Chileno has contemplated, "llámame" is an imperative, so you'd have to say rather:
Llámame cualquier día. -> Call me any day.
Llámame en cualquier momento. -> Call me any time.
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