Validation Rule for Phone Number

Hi, 

I created a validation rule for Phone field which

1. Should only contain numbers(no special characters# or text)

2.Should only contain 10 digits

OR( NOT( (LEN(Phone) == 10)), !REGEX(Phone, "[0-9]*") )

3.User should also be able to save record even if Phone field is blank------> This is causing error. How can I achieve this?

Thanks