asterisk users - Oct 2005 - Error: -- Got SIP response 481 "Call Leg/Transaction Does Not Exist" back from xxx.xxx.xxx.xxx

Hi,

I am running asterisk on Fedora Core 3, Configured few extension,  I receive
frequent error message on * console as

-- Got SIP response 481 "Call Leg/Transaction Does Not Exist" back
from
xxx.xxx.xxx.xxx.

This only comes from two extensions which I configured

Any idea what does this error means


Regards

Shaikh