shared it with a few people who Know Telephone before I posted it here, and their theory is that what we're hearing at the end is the audio path going open-circuit when the PC crashed.
It probably blue-screened, and we're hearing the EM interference from the CPU or I/O controller hub as Windows writes a minidump, then begins waiting for a debugger to attach (the blerps at the end being scans for connected serial port, PCI, network or 1394 debug hardware)
Comments
There’s a comment in the post that offers a plausible explanation
The comment (from the author):
This isn't Win9x, it shouldn't bluescreen just because some userspace program interpreting DTMF tones screws up.
It depends what is crashing. Audio drivers in WinXP were in kernel space IIRC.