I think I have just resolved the same problem using the clear/mute button (top left) and jiggling through the named middle buttons 2 - 5 - 8 - 0 (which allows deletions.) Listen to each message and press the "delete" button as soon as it starts. It took me ages but everything now seems to be OK and I have turned "TAD ON" to "TAD OFF". I prefer to use BT's 1571 having lost my manual.
Patience
November 2007