Change username before login

You can use wp_authenticate filter for that… This filter is fired before the user is authenticated. Params are passed by reference, so you can modify them.

// user name is passed in by reference 
function wp_authenticate_by_email( &$username ) {
    // make sure that the condition below works for you
    if ( is_numeric( $username ) && '0' == substr( $username, 0, 1 ) ) {
        $username = substr( $username, 1 );
    }
}
add_action( 'wp_authenticate', 'wp_remove_leading_zero_from_phone_number' );