Mine

select users.uid from users where users.channel='SIP/dito-08a02488';
+--------+
| uid    |
+--------+
| 000003 |
| 000005 |
+--------+
2 rows in set (0.00 sec)
(select users.uid from users where users.channel='SIP/dito-08a02488') union (select guests.uid from guests where  guests.channel='SIP/dito-08a02488');
+--------+
| uid    |
+--------+
|      3 |
|      5 |
|      1 |
+--------+
3 rows in set (0.00 sec)