Japanese Telephone etiquette

A quick question for anyone who knows Japanese:

I'd like to say "I'm sorry, but you have the wrong number" very politely to a total stranger over the phone, and I'm not quite sure how.

Would it go something like:
すみませんが、電話番号をお間違いをしたかも知れませんけれども。。。(?)

Does anyone know the standard, set phrase that's used in Japan?

Thanks very much in advance!