Short answer: it depends.
At first I would say it is an attribute but…
if you need to:
– have many different kinds of phone numbers and you cannot say how many (0-n) and maybe the type of the number varies in different cases (home number for a person, office number for a company, fax number for both) and maybe the types for phone number do not remain the same
– have history data for the numbers (which number was valid at certain time)
or something similar.
You might actually have an entity in hands.
Or if you know there might be max four phone numbers but two of them are mandatory, we come back to the conclusion it is an attribute.
So it depends on the requirements weather phone number is an attribute or an entity.