Motorola MD4260 (by 2 users)

I had the same problem.
Here is the solution that I received from Motorola tech support:

1.Delete all messages
2. Unplug the power and re-apply power
3. Quickly press and release << (previous message) and >> (next message) buttons together while the "Press review to receive setup instructions" message is playing.
The display will change from a segment scanning clockwise to anti-clockwise
4. Quickly press and release the On/Off button before the "Press review to receive setup instructions" ends. The display will change to a square scanning clockwise.
Note, this goes by quickly. I had to repeat steps 1-4 a couple of times before I got it right.
5. Press and release the delete button. The unit will emit a beep.
6. Press and release the on/off button. The unit will emit another beep and annouce "Press review to receive setup instructions"
Done!