File 300 contains a phone number where one to three of the digits have been replaced with a placeholder (‗X‗). Using your modem, dial all possible phone numbers that could be generated by replacing the placeholders with the digits 0 through 9. When you discover a valid phone number that connects to another host, append it to file 301.

Note that the phone numbers must be dialed and appended in a specific order such that a placeholder is only incremented when the placeholder to the right cycles through all possible values from 0 to 9 and is reset to 0.

For more information see "Hacker Skills: Modem Control at the Direct Level" in the second issue of the zine.