The winning line-up was a no brainer. Didn't need any test equiptment for this experiment. Taking last place, was Ma Bell, which sounded like an old telephone... a little thin, a bit tinny, nothing to write home about.
Second runner up: The Panasonic: Clear, though slightly muddy. Some would say great, until you hear the Telo Handset.
In first place, easily: The OOMA Telo, despite the fact I don't like most of the layout of the phone, the audio was magnificent.
Make 3 recordings yourself.. and test your friends ears. You would be pleased at how well you sound "on the phone"......
Which handset did you use on which transport to leave messages on your Ooma number and what method did you use to listen to your messages? Also which codec were you using? Since DECT 6 uses the g.726 codec between the handset and the base I have a difficult time believing that the Panasonic beat out the corded Ma Bell phone, many Panasonic models are cat-IQ compliant which would mean they use the g.722 codec between the base and the handset so they should at least inject no quality loss itself if your model is one of those. The Telo handset if actually using the cat-IQ standard DECT should at least come close to equaling a corded phone as well but not beating it while using the iLBC codec.
The only thing I can think of is that the quality of the Ma Bell phone itself as you state must be very low. If you did not force the g.711 codec to be used you cannot achieve full 100% toll grade quality with any handset even though the MOS of iLBC is not too much lower than that of g.711 while using a lot less bandwidth and is more tolerant of packet loss. I've seen a lot of exaggerated claims of sonic nirvana forcing the g.711 codec but it still sounds like a phone call.
There is also the question of whether your call actually went over the PSTN to be recorded. If you want to be unscientific about it but at least be somewhat meaningful, call and leave messages from all of them on voicemail elsewhere and listen then. From your Telo handset directly to Ooma's voicemail on the server is not a good comparison.