How to create a clickable static call us button that links to a new setting in general

I recently had an admin setting created on my website that allows me to input a contact number to display on the front end via a shortcode.

Now I am now trying to use this field to also create a clickable and static call button for mobile devices only that appears on the bottom of their screens.

There are plugins like WP Call Button that achieve this, but I am trying to keep my website as light as possible. Any help would be much appreciated!

I think I may need another 2 admin fields created that act as a text label as well as the hyperlink function? e.g. tel:035555555?

Here’s the code that was created (thanks again Walter) for my general settings:

/**
 * Class for adding a new field to the options-general.php page
 */
class Add_Settings_Field {

    /**
     * Class constructor
     */
    public function __construct() {
        add_action( 'admin_init' , array( $this , 'register_fields' ) );
    }

    /**
     * Add new fields to wp-admin/options-general.php page
     */
    public function register_fields() {
        register_setting( 'general', 'phone_number_custom', 'esc_attr' );
        add_settings_field(
            'custom_phone_number',
            '<label for="custom_phone_number">' . __( 'Phone Number' , 'phone_number_custom' ) . '</label>',
            array( $this, 'fields_html' ),
            'general'
        );
    }

    /**
     * HTML for extra settings
     */
    public function fields_html() {
        $value = get_option( 'phone_number_custom', '' );
        echo '<input type="text" id="custom_phone_number" name="phone_number_custom" value="' . esc_attr( $value ) . '" />';
    }

}
new Add_Settings_Field();

How to create shortcodes that pull custom field data from general settings