Phone number matching for PHP regular expression

Preg_match_all ("/(?)  (D{3})?  )?  -? (? (1) [-S]) d{3}-d{4}/x ",
"Call 555-1212 or (221)-(820) -555-1212", $phones);
Print_r ($phones [0]); echo "<br>";
Print_r ($phones [1]);

The results of the output are:
Array ([0] => 555-1212 [1] => (820)-555-1212)
Array ([0] => [1] => 820)
Who can help explain the meaning of this regular expression?  The key is (?)  (D{3})? )? (? (1) [-S]) this part.
Why can match (820), but not match (221)?

(?  (D{3})? )? (? (1) [-S])
(1) The Reference (D{3})
(? (1) [-S]) if (d{3}) match succeeds, continue matching [-s]
The prototype is (? A b|c), if a matches, continue to match B, otherwise match C