Has anyone played Kagi nochi tobira?

The only ones I had trouble with were 8 and 14...

Playing #2 now.

Mind helping me out with 19 or 20? I can't figure 'em out.

 Room 19

I can make the door appear with 2+6=9, but I can't figure out how to get the key to appear.

 Room 20

I found that you can press the bottom-left button on the button pad to make symbols appear in a sort of phone-formation. I haven't figured out how to apply it, though.