I think it was pretty difficult for most people to tap in accurately a seven digit number. If you're timing was the least bit off any part of the way you had to start over. You also risked discovery while doing it.
It wasn't that hard to tap in zero and get the operator to do it, though, but I think a lot of people didn't think of that option.
All in all I'd say the dial locks, which were inexpensive, were reasonably effective to protect against telephone abuse.
I've only rarely seen plate locks to cover up a Touch Tone dial.
Years ago our Centrex required a PIN to make long distance calls after hours.