Maybe you need to aggregate?

SELECT

CALL_1,

MAX(CASE WHEN RESP.OUTCOME1 = OUT.OUTCOME THEN OUT.SIMPLIFIED END) AS OUTCOME_1,

CALL_2,

MAX(CASE WHEN RESP.OUTCOME2 = OUT.OUTCOME THEN OUT.SIMPLIFIED END) AS OUTCOME_2,

CALL_3,

MAX(CASE WHEN RESP.OUTCOME3 = OUT.OUTCOME THEN OUT.SIMPLIFIED END) AS OUTCOME_3

FROM RESPONDING_CALLS RESP

LEFT JOIN OUTCOME_SIMPLIFIED OUT

ON RESP.OUTCOME_1 = OUT.OUTCOME

OR RESP.OUTCOME_2 = OUT.OUTCOME

OR RESP.OUTCOME_3 = OUT.OUTCOME

GROUP BY

CALL_1,

CALL_2,

CALL_3;