Revoke an unredeemed connection code

Mint a six-digit code that connects one app device (laptop or mobile) to this line. A SIP credential carrying an extension IS a person's line; the desk phone registers with its SIP password, while app devices attach with a code instead. The plaintext code is returned ONCE and cannot be retrieved later. It is also emailed to the address supplied, so an agent with no company email address can still be enrolled by reading the digits out. Single use, ten minutes, and issuing a new code revokes any outstanding one for the same line. Fails with `numbers.extension_required` when the line has no extension — a device enrolled onto an undialable line could never be reached.