Could you repeat that?
There's no real special phrase for this situation, unlike English's "I beg your pardon". Therefore, anything conveying the sense of "please repeat" or "I didn't hear" works. For instance, as is increasingly the case in English as well, normally you could just say "what?", 什麼?. 

To be a little more preciser, as well as polite / less familiar, you could say 對不起, 我沒聽清楚, "Sorry, I didn't quite catch that." And then you can ask them 可以再說一遍嗎?, "Could you repeat that?"

Really, just about any translation of what you'd say in English would work. Since there's no specific phrase, as long as the sentence is grammatically sound and makes sense, you'll be understood just fine.