Wardialing is an information reconnaissance technique to find phone numbers of interest within a list. Dial up every number in an area code and listen for modems, fax machines, or bulletin board systems. Phone numbers can have a wide variety of systems behind them, but aside from phone books there’s no record of what these might be. Part of that is because the only way to know.. is to call.

Valtteri Lehtinen called nearly 60,000 numbers in Finland to understand the telephone network better. This was done via a VoIP trunk to make simple calls over GSM - cellular connections. Once a call was connected, they recorded 60 seconds of audio for classification then hung up. They tried to focus on only public numbers and ignored any premium numbers that would have run out their calling credits too quickly.

Over the course of 40 days they found that only 3% of calls were answered, and only 70% were interesting. There were only 74 unique and interesting responses. These ranged from machines, faxes, and systems for presenting information to callers.

One was a message with the following:

Welcome to the end of the world.
There is still some hope left.
If you want to be rescued then press 1.
If you want to join the zombie army then press 2.
Choose something quickly, we don't have all day.
...
You have chosen to be a zombie and join the zombie army.
Please wait patiently to be bitten.
Please do not call again.
Thank you for calling.