Wikipedia
Dial-A-Joke (or joke line) refers to a telephone service that users can call to listen to previously recorded jokes. Jokes are recorded on an automatic answering machine. In the past, many jokes were recorded on cassette tape and then played sequentially, each caller hearing the next joke on the tape. Modern touch tone phones allow callers to select different joke types: knock-knock, joke of the day, professional humor, random, etc.
Dial-A-Joke operators will sometimes actually answer the phone, which is called taking a live call. The co-founder of Apple, Steve Wozniak actually met his first wife by taking a live call on his Dial-A-Joke service. He started the service as a hobby in 1973, at which time it was the first Dial-A-Joke in the San Francisco Bay Area. Wozniak's Dial-a-Joke line received 2,000 calls a day during its three years of service.