Recently the voicemail banner quit displaying on all three handsets. If I reset (popout/popin battery) on one handset the voicemail banner will display on that handset only. So I reset all three handsets and I listen to the voicemail and delete it and the voicemail banner is still there. If I reset that handset the voicemail banner goes away on that handset only.
Next I tried resetting everything by removing power from the Telo and popping batteries on all handsets, waiting 5 minutes while handsets were displaying "Scanning for Ooma base unit" and then restored power to the Telo. Then I called the Ooma from my cell phone and left a voicemail and all three handsets displayed the voicemail banner. I listened to the voicemail on one of the handsets and deleted it. The voicemail banner on all three handsets turned off.
Conclusion: Resetting everything fixed the problem.