forgot my voicemail pass word, how do I reset it needs my old pass word?

I have a new 4s iphone

I sync it with my computer from the old 3g phone.

I want to set up my voice mail on the new 4s phone and it keeps asking me for my voice mail pass word

I don no remember what if any that was.

How do Iset up my voice mail without knowing my old password for voice mail? how do I find out my old voicew mail pass word?