Read and tried many of the attempted fixes (hours and hours) with no joy. Replaced the router with a new 2-unit (parent-child) mesh system to improve coverage. No joy.
Hope this helps someone else: failing to resolve the problem as described above, decided to replace the Telo with a new Telo Air 2 with WiFi. This fully resolved the problem. Calls are no longer dropped, the connection is now by WiFi from the router. Fewer wires. Call and sound quality is also much improved over that achieved with the replaced old Telo.
I should have paid more attention to the old Telo's faint blue center main light, which had become barely perceptible over time, especially recently. The new Telo Air 2's center blue light is as bright as the old Telo's when it was first set up years ago.
I suspect that the Telo power supply doesn't last forever, and that the failing blue light on the old Telo may have been trying to tell me something.
Maybe not a solution for everyone, but hope it helps others encountering a similar problem.
BTW: Ooma customer service (about 30 minutes total: reaching them, being referred to the tech team, etc. ) was responsive and helpful in adding the new Telo Air 2 to my account, plus suggesting my using the phone app to complete the setup, which worked smoothly.
Best Wishes,