Hello guys, I need your help.

I need optimize this tables in query join and tried the syntax EXPLAIN SELECT.

My answer is: why don't show in the columns 'key' the key 'tel,action' assigned in the table 'tbl_A'?
According to the output of EXPLAIN SELECT how to optimize my tables?

Can you help me?
Thank you in advance.
Code:
mysql> EXPLAIN SELECT
	u.tel AS tel,
	CASE
WHEN action IS NOT NULL THEN
	CONCAT(
		'***',
		u.tel,
		'***',
		' ',
		u.action
	)
WHEN u.Notes IS NOT NULL THEN
	CONCAT(
		'***',
		u.tel,
		'***',
		'Notes'
	)
ELSE
	u.tel
END AS tel_1
FROM
	tbl_A u
JOIN tbl_B ON u.tel = tmp.tel
WHERE
(
	u.action NOT IN ('Regular')
	OR u.action IS NULL
)
GROUP BY
	tel
ORDER BY
	tel ASC;
+----+-------------+-------+------+----------------+---------+---------+----------+-------+----------------------------------------------+
| id | select_type | table | type | possible_keys  | key     | key_len | ref      | rows  | Extra                                        |
|  1 | SIMPLE      | u     | ALL  | tel,action     | NULL    | NULL    | NULL     | 22955 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | tmp   | ref  | tel            | tel     | 258     | db.u.tel |    55 | Using where; Using index                     |
+----+-------------+-------+------+----------------+---------+---------+----------+-------+----------------------------------------------+
2 rows in set

mysql>