Cast cc_call.destination as CHAR to allow the use of indexes

Before:
mysql> explain SELECT * FROM cc_call,cc_prefix WHERE card_id = 64 AND cc_call.destination = cc_prefix.prefix ORDER BY starttime DESC LIMIT 0,10;
+----+-------------+-----------+------+---------------+------+---------+------+----------+----------------------------------------------+
| id | select_type | table     | type | possible_keys | key  | key_len | ref  | rows     | Extra                                        |
+----+-------------+-----------+------+---------------+------+---------+------+----------+----------------------------------------------+
|  1 | SIMPLE      | cc_call   | ALL  | NULL          | NULL | NULL    | NULL | 17432949 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | cc_prefix | ALL  | PRIMARY       | NULL | NULL    | NULL |    17174 | Using where; Using join buffer               |
+----+-------------+-----------+------+---------------+------+---------+------+----------+----------------------------------------------+
2 rows in set (0.01 sec)

After:
mysql> explain SELECT * FROM cc_call,cc_prefix WHERE card_id = 64 AND cast(cc_call.destination as char) = cc_prefix.prefix ORDER BY starttime DESC LIMIT 0,10;
+----+-------------+-----------+--------+---------------+-----------+---------+------+------+-------------+
| id | select_type | table     | type   | possible_keys | key       | key_len | ref  | rows | Extra       |
+----+-------------+-----------+--------+---------------+-----------+---------+------+------+-------------+
|  1 | SIMPLE      | cc_call   | index  | NULL          | starttime | 4       | NULL |   10 | Using where |
|  1 | SIMPLE      | cc_prefix | eq_ref | PRIMARY       | PRIMARY   | 60      | func |    1 | Using where |
+----+-------------+-----------+--------+---------------+-----------+---------+------+------+-------------+
2 rows in set (0.01 sec)