flash_call — the code is in the incoming number

We place a dropped call to the user's number, and the code is the trailing code_length digits of the incoming number. The user reads them off the screen and enters them on your side.

What to watch for

• Tell the user what to enter. "The last 4 digits of the number that called you" — without that hint conversion is noticeably below SMS.
• The call does not need answering; it drops by itself.
• Not to be confused with reverse_flash_call, where the user places the call and no code is typed at all.