I GOT IT! It was 0x3F.
It pays to read the datasheet, as people would say.
A common mistake for beginners is their unwillingness or inability to read datasheet and they keep trying to get this thing to work, only in vain. A datasheet is like a map to a maze. The chance of a blind person working out of a maze without a map is practically zero.
BTW, 0x00 is the general call address for i2c devices.