Callback verifier
A BBS feature that hung up and dialled the new user's stated number back before granting the account full access.
New users entered their phone number at sign-up. The callback verifier — built in, or bolted on as a door — dropped the call, dialled that number and asked the user to log in again. If a modem answered, the access level went up automatically. It killed off made-up accounts and gave the sysop a number to ring when someone misbehaved.
The scheme had holes. Long-distance calls cost the sysop money, so out-of-area users often stayed unverified, and phreakers quickly learned to give the numbers of payphones or forwarded lines. On elite boards it was still a common first filter, ahead of user voting and personal vouching.