lrm_save_custom_field

login_via_phone.php

<?php
// ONLY IF you want the user login via phone-number
// COPY after

add_filter('lrm/login_info_filter', function ($info) {
  $user_login = $info['user_login'];
  // Optionally
  // $user_login = str_replace(["+", "-", " "], '', $info['user_login']);  

  // Commnets this to allow phone duplicates
  $users = get_users(array(
      'meta_key'     => 'phone',
      'meta_value'   => $user_login,
  ));   
  
  if ( $users ) {
    $info['user_login'] = $users[0]->user_login;
  }
  
  return $info;

});

lrm_save_custom_field.php

<?php
// How to add html: https://monosnap.com/file/2MVkjClgYE3W3U9cgPEjy4VXXCzXf2
// COPY AFTER
add_filter( 'registration_errors',function ( $errors, $sanitized_user_login, $user_email ) {
    if ( empty( $_POST['phone'] ) ) {
        $errors->add( 'phone_error', __( '<strong>ERROR</strong>: Please enter your Phone.', 'LANG_DOMAIN' ) );
    } else {
        // Commnets this to allow phone duplicates
        $users = get_users(array(
            'meta_key'     => 'phone',
            'meta_value'   => $_POST['phone'],
        ));   
        
        if ( $users ) {
            $errors->add( 'phone_error', __( '<strong>ERROR</strong>: This Phone is already registered!', 'LANG_DOMAIN' ) );
        }
                        
    }
    return $errors;
}, 10, 3 );

add_action( 'user_register', function ( $user_id ) {
    if ( ! empty( $_POST['phone'] ) ) {
        update_user_meta( $user_id, 'phone', sanitize_text_field($_POST['phone'] ) );
    }
});

show_in_admin.php

<?php
// COPY AFTER


/**
 * Back end display
 */
add_action( 'show_user_profile', 'lrm_show_extra_profile_fields' );
add_action( 'edit_user_profile', 'lrm_show_extra_profile_fields' );

function lrm_show_extra_profile_fields( $user ) {
    ?>
    <h3><?php esc_html_e( 'Personal Information', 'cn' ); ?></h3>

    <table class="form-table">
        <tr>
            <th><label for="phone"><?php esc_html_e( 'Phone', 'LANG_DOMAIN' ); ?></label></th>
            <td><?php echo esc_html( get_user_meta( $user->ID, 'phone', true ) ); ?></td>
        </tr>
    </table>
    <?php
}