When we get a new voicemail message, the tiny "Play" button slowly flashing orange is impossible to see at a distance, and too low-key of an indicator. I've not noticed voicemails for a few days myself.
To me, it seems like it should be an easy firmware update to flash the large, blue ooma logo on the Telo to make a waiting voicemail indicator much more noticeable. I know there are at least 3 colors possible that I have seen: red, purple, and blue (and maybe others). You may still want to flash the "Play" button, just to guide the customer on what button to press to get the voicemail messages.
Also, an additional feature that should certainly be possible via a firmware update: Have the ooma Telo (and/or Telo Handset) "chirp" once a minute to indicate a new, unplayed voicemail message. Whether the "chirp" is played (or not) should be configurable on the setup.ooma.com Setup screens.