OK, I just came from an electronics forum, and found that messages from digital machines can be recovered. I don't know about 1992, but here's how one guy did it in 2016.
Success! The messages have been recovered. Through a friend of Karlin‘s I got to work on this project and I will post a quick summary describing the procedure we followed.
open the answering machine and desolder the AT45DB321B flash memory chip.
The flash chip has an SPI interface; we connected it to the GPIO of a Raspberry PI to clock out all the data and save it to a file using a Python script to bit-bang the data out.
The Python script essentially sent the following bytes: [0xe8, 0x80, 0x04, 0x01, 0x0, 0x0, 0x0, 0x0] which tells the flash chip to send us the data in continuous mode starting at the first register in the first page of memory. After that, each cycle of the clock outputs the next bit from the chip for the entire contents of the chip (4,325,376 bytes specifically) before rolling over and starting at the first register again.
The audio format seems to be a form of the G.721 format.
Install Vox Studio by xentec(I can't post links so google for it) and open the file as a Elan Informatique 32K ADPCM @ 8000 Hz sampling frequency then save as a WAV or whatever floats you boat.
If you don't want to pay $500 for Vox Studio you can do this, download g72x++.tar.bz2 (I can't post links so google for it) unpack and compile with build.sh. Use it with this string of commands in Bash:
cat <Raw data file> | decode-g72x -64 -u -L | sox -r 8000 -b 8 -c 1 -e u-law -t raw - -t wav <output file> ; cat <output file> | sed 's/\x0\x0\x0\x0//g' > tmp ; mv tmp <output file>
Now, the question is: did Sherrell have a digital machine or a tape machine?