My recollection is we were specifically curious about what today would be called invalid address space patterns, aka numbers that were outside the patterns used in valid phone numbers at the time (such as area codes could have a 1 in the middle digit but prefixes could not have a 1 in the middle in the 1980's).
We pulled out a paper phone book, devised a set of criteria for potentially interesting superficially invalid phone numbers, and started dialing them. On a rotary dial phone. And we found one pretty quickly. And it was a much scarier experience than we had expected.
The exact number we dialed is unfortunately(?) lost to the passage of time.
Here's a fun one: next time you get a new phone number (USA), ask for it to end in "9999".
It's possible, but not without social engineering (or influence?).
We pulled out a paper phone book, devised a set of criteria for potentially interesting superficially invalid phone numbers, and started dialing them. On a rotary dial phone. And we found one pretty quickly. And it was a much scarier experience than we had expected.
The exact number we dialed is unfortunately(?) lost to the passage of time.