why it uses ´Wie´ instead of ´Was´ in this sentence?
Just a German thing I am afraid. Literally: "How is your wife's number?"
so if I substitute 'wie' with 'was', is it still appropriate here?
I don't think so. Just like "How is your wife's number?" does not sound right in English. I believe that "Was ist die Nummer deiner Frau?" sounds wrong in German.
"What is your wife's number?" is very ambiguous in English. Is this question asking for the wife's age?