Difference between 'once' and 'after'

[Daemon99]

Can some one please let me know if there is any difference between the two sentences below?

1. I am going to breakfast. I'll join the call after I am back.

2. I am going to breakfast. I'll join the call once I am back.
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Non-native speaker of English
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I intend to live forever - so far, so good.

[Mister Micawber]

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I suppose that 'once' expresses a bit more immediacy in joining the call...whatever 'join the call' means.
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Native English teacher at Mister Micawber's