Make Email address field optional

In my business setup, it it not a MUST for customers to have email addresses. Phone numbers are enough.
So I would like to create a mandatory 'Username' field in the Signup page and make the 'Email address field' optional. Customers sign in with their usernames & passwords. Any communication with them is through Text messaging or Phone call.
Please guide on how I go about this.

Ouch, email is the very core component that relies on the customer. Unique users, password hashing, and everything.

It's just too much to list but I would say search all files for the occurence of 'email' and 'customer_email'. And start your work turning them into username from there. You will see the form component says form_draw_email_field() you would wither turn that into form_draw_username_field() which is a function you create in func_form.inc.php, or use a standard form_draw_text_field().

Maybe you can use an e-mail internally say username@localhost but the user only sees username in the field. Meaning you add @localhost to their passed form data.

Thanks for the hint. Ill go by the first suggestion because the second one eliminates email functionality completely from the whole app.
It's only the login page I wd like to replace email with username. In the signup page or customer details page, email field should be there as an optional field for the customers who have it, not marked as 'Required'. Gonna start my customization straight away.