Sidetone is the response to Entry #36.
Its description is "# 000-0000".
The audio is a bit distorted version of Alex's voicemail ("You have reached the voicemail of Alex Kralie. I'm not answering my phone right now, please leave your name and number and I'll get back to you as soon I can"), so whoever made this video has his phone number.
This video contains several frames of what appears to be footage of Alex from Entry #47, which wasn't uploaded until a lot of months later, so this means that whoever uploaded this has the complete access to Jay's tapes and files on his laptop.
Also, the four numbers that appear near the end decode to "LIES". Probably this is a reference to the fact that Alex intentions aren't good or that "I'll get back to you as soon as I can" is a lie, and Alex won't going to call back Jay.
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Guide to all ToTheArk's videos
Non-FictionA little guide to watch and to understand all the ToTheArk's videos, a part of the horror web series Marble Hornets. WARNING: it is necessary have watched Marble Hornets, because it contains important spoilers about the plot.