Oracle 18c

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create table mytable (
    id int, 
    phone1 int, 
    phone2 int,
    check (phone1 <> phone2)
);
create unique index myidx on mytable(
    greatest(coalesce(phone1, phone2), coalesce(phone2, phone1)),
    least(coalesce(phone1, phone2), coalesce(phone2, phone1))
);
insert into mytable values(1, 111, 111);
ORA-02290: check constraint (FIDDLE_SMBYKTEIHNNVOHKZSCYK.SYS_C0020876) violated

begin
    insert into mytable values(1, 111, null);
    insert into mytable values(1, 111, null);
end;
/
ORA-00001: unique constraint (FIDDLE_SMBYKTEIHNNVOHKZSCYK.MYIDX) violated
ORA-06512: at line 3

begin
    insert into mytable values(1, 111, null);
    insert into mytable values(1, null, 111);
end;
/
ORA-00001: unique constraint (FIDDLE_SMBYKTEIHNNVOHKZSCYK.MYIDX) violated
ORA-06512: at line 3

begin
    insert into mytable values(1, 111, 222);
    insert into mytable values(1, 222, 111);
end;
/
ORA-00001: unique constraint (FIDDLE_SMBYKTEIHNNVOHKZSCYK.MYIDX) violated
ORA-06512: at line 3