Add field to ‘Add User’ admin page

Here’s the way I have always done it. All of this code goes in functions.php. Of course this adds a whole new section to the user profile.

add_action( 'show_user_profile', 'be_show_extra_profile_fields' );
add_action( 'edit_user_profile', 'be_show_extra_profile_fields' );

function be_show_extra_profile_fields( $user ) { ?>

    <h3>Extra Contact Information</h3>

    <table class="form-table">
        <tr>
            <th><label for="contact">Contact Number</label></th>
            <td>
                <input type="text" name="contact" id="contact" value="<?php echo esc_attr( get_the_author_meta( 'contact', $user->ID ) ); ?>" class="regular-text" /><br />
                <span class="description">Please enter your phone number.</span>
            </td>
        </tr>
    </table>
<?php } 
add_action( 'personal_options_update', 'be_save_extra_profile_fields' );
add_action( 'edit_user_profile_update', 'be_save_extra_profile_fields' );

function be_save_extra_profile_fields( $user_id ) {

    if ( ! current_user_can( 'edit_user', $user_id ) ) {
        return false;
    }

    update_usermeta( $user_id, 'contact', esc_attr( $_POST['contact'] ) );
}

Alternatively If you want to add some contact methods to the already existing section in the user profile you can use the following and it’s a little cleaner. Below is the function i usually use to get rid of outdated social platforms and add some new ones for users.

add_filter( 'user_contactmethods', 'be_hide_profile_fields', 10, 1 );
function be_hide_profile_fields( $contactmethods ) {
    unset($contactmethods['aim']);
    unset($contactmethods['jabber']);
    unset($contactmethods['yim']);

    $contactmethods['twitter'] = 'Twitter';
    $contactmethods['facebook'] = 'Facebook';
    $contactmethods['linkedin'] = 'LinkedIn';
    return $contactmethods;
}

If you wanted to add a phone number to that you could just add something like

$contactmethods['phonenumber'] = 'Phone Number;

to the function above and it will create the field for you.