Sidetone was posted on March 15th, 2011.
It has the description "# 000-0000".
In total, this clip is 0:18 long.
Speculation & Analysis
The numbers shown on the numberpad were obviously trying to focus on the 0 "OPER" as a reference to The Operator.
The background throughout the majority of the footage appears to be some kind of furniture, going by the stitch marks at the top of the frame. The change of lights and shadows over it seems to me to be possibly someone walking around in front of a light source.
There are quick cuts of figures, looking almost like the silhouette seen at the end of Entry 29.
The alphanumerical subsitiution for the numbers shown on-screen would be 12= L, 9 = I, 5 = E, 19 = S
Perhaps TTA is trying to tell Jay that Alex being unavailable is a lie in an attempt to get Jay to contact him
Possible secondary theory:
I'm pretty sure the video is just TTA going, "Hey, J? So, yeah, Alex, he's never going to call you back."
What with the word LIES after the voicemail says "And I'll call you back as soon as I can.", and then the brief frames of the Slendertunnel where in the previous entry we saw a rock with blood on it.
So either Alex is really dead, which I think is more likely given the evidence, or he's evil/insane and wasn't going to actually help J. ~Owleyet from Unfiction forums
This seems somewhat likely to me, depending on the timeline of when the footage from 29 and 36 were taken ~ Ghost1807
I noticed that not all of the figures that appear are of the Operator. Some appear to be of Alex from Entry 22. ~ haroldoftherocks