Hi, How do I make my site that email is not required for only users who want to post items (without having an account)

Osclass require mail, means only way is to hide it and populate with some generated value.
Maybe based on phone number

Is there an add-on to do this, changing the email with the phone number?

No,but there is Phone number login plugin that can help you to get uniqueness of phone nunbers on user accounts and provide login using phone number.