Ruby on Rails

Hello. I know the title may be broad, but I am not sure how else to put it.

I am writing a model, in which the attribute "number_of_extensions" needs 2 things:
1. It is required for this model
2. It must be a whole, positive number

I know I can do this simply by

validates :number_of_extensions, presence: true, numericality: { only_integer: true }

However, if I validate for JUST numericality, is it assumed that the attribute SHOULD be present?

Also, while I am talking about validations, I have another question:

I know that the following type of validations is not in the docs anymore, but it is still supported. Would the following line be possible?

validates_numericality_of :number_of_extensions, :number_of_sidecars, :number_of_phones, only_integer: true