Das erste Sub-SELECT könnte dann vielleicht so aussehen:

SELECT MIN(TEL_NUMBER) FROM R3P02DATA.ADR2
WHERE ADDRNUMBER=A.ADRNR AND R3_USER = '1' AND CONSNUMBER =
(
SELECT MIN(CONSNUMBER) FROM R3P02DATA.ADR2
WHERE ADDRNUMBER=A.ADRNR AND R3_USER = '1'
)