You could try performing a factory reset on the Telo, this will return all settings to default and wipe the previous handset registration, so re-registering the handset may then work.
1. Unplug the Telo power cord for 30 seconds, then plug back in.
2. As the Telo begins its startup, the 1 & 2 buttons will light up for a short while, followed briefly by the trash and envelope buttons, and then the bottom row of buttons will begin to flash in a scrolling sequence, which will continue approximately 28 times.
3. At the end of the blue flashing bottom row period, the Ooma logo will begin to flash red - IMMEDIATELY hold down the "stop" and "trash" buttons and the Ooma logo will alternatively flash red and blue. Continue to hold down the "stop" and "trash" buttons until the logo turns solid red and then goes out. After the logo goes out release the buttons and the Telo will restart.
You can see a poor quality video of the proceedure here:
http://www.youtube.com/watch?v=QNW7BqAftQw