I take it you mean the rmessage bank is full.
Did a search can't find one of these so here is only a guess going by my BT phone.
Try press the button that brings up the message but hold it down and it may take you to message archived and tells you how many in there, release the button. Now if it doesn't auto start announcing the messages press the message button again and release wait for and delet all messages as they come up even halfway through, press delete button. Most phones have a 'delete' all button job done.
bob, February 2013
