You were talking to someone and you guys took a break. After coming back, you find that other person is unavailable.
Which is more appropriate to say?
Call me once back.
Call me when you are back.
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"Call me when you are back" would be more natural during speech.
"Call me once back" looks like something that would be written in an email, by someone who is being terse and writing as few words as possible. Which is fine if that is your style.
In speech you would say "Call me once you're back."