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She might have called while I was out.
Literal
She [topic-は] I [subject-が] going-out is [during-間に] phone [object-を] gave [might-かもしれない].
~ている間に ('while X is/was [in a state]') marks a time interval during which something else happens — 'while I was out, [something happened].' The 間に sharpens the timing: the embedded action falls *within* the interval, not just *during* it. 電話をくれる ('to give [me] a phone call') uses くれる ('give to me/us') to mark the speaker as recipient — a phone call as a kind gesture. ~かもしれない hedges the whole claim into a guess.