Both sentences are polite form. But number1 is not used for usual conversation. '입니까' is used for very official conversation and document. If you use that sentence for casual conversation, it will give an unnatural impression. Number2 is more suitable. And you can use this when you talk with older people or same age(but not close relation). The most casual form is '휴대폰 번호 뭐야?' or '휴대폰 번호가 어떻게 돼?'