I didn't know his telephone number．________ it, I ________ then．

If I had known; would have rung him up

Should I know; would have rung him up

If I knew; would ring him up

Had I known; would ring him up

I didn’t know his telephone number．_________ it, I _________ then．

Had I known, would ring him up

Should I know, would have rung him up

If I knew; would ring him up

Had I known; would have rung him up