Marie, I'm not sure what your create_form_input function does (and I'm confused about some of your HTML), but the following is a very simple works-out-of-the-box demonstration of how to handle radio button logic.
The first thing you should do is test whether any radio buttons are selected or not with the isset function.
The value attributes of the input elements in the HTML form are the possible values that can be stored in the $_POST['consent'] array element when a radio button is selected.
NOTE: Obviously you have to have a proper HTML doctype, opening and closing body tags etc.
The HTML in this example is only the form portion of the page.
Set up an array of valid values and check to make sure the value passed in is one of those valid values.
Since this field isn’t required unless the phone number is filled in, save its check for later.