Mask more digits

Change the number as your demand and use same number of characters to mask it.

add_filter( 'rtcl_phone_number_placeholder', function () {
	return 'XXXXXXX';
} );

add_filter( 'rtcl_phone_number_options', function ( $phone_options, $data ) {
	if ( isset( $data['phone'] ) ) {
		$phone                         = $data['phone'];
		$phone_options['safe_phone']   = mb_substr( $phone, 0, mb_strlen( $phone ) - 7 ) . apply_filters( 'rtcl_phone_number_placeholder', 'XXXXXXX' );
		$phone_options['phone_hidden'] = mb_substr( $phone, - 7 );
	}

	return $phone_options;
}, 10, 2 );