Where do i find my mail box number to set up my voicemail on my iphone?

Yes, take MaryAnn's advice as the voicemail number is definitely NOT your phone number. Every voicemail platform has a unique deposit and retrieval number depending on the market you are in. Nobody would be able to give you that number except for somene with the same area code and exchange as your number so the best thing to do is call customer service at the number she gave you.