Usually this phrase is used to respond to someone who has said “I have something to tell you.” (話があるんだ＝あなたに話したいこと[or 話さなければならないこと] がある。)
“話” = "あなたに話したいこと（something to tell you）”
「話ってなんだい？」means "So what is it?"
(You can even omit なんだいfor this. )
Hope it makes sense:)
November 14, 2018
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