Problem with Nortel i2004 UNISTIM Phones

i had configured nortel i2004 phones with asterisk 1.6 and they were working properly. Yesterday i switched on the phones and something strange happened. the phone registers with the server but it is not accessible from another phone. The asterisk CLI displays the message "Call from ‘5802’ to extension ‘7625’ rejected because extension not found in context ‘incoming1’. " Though everything is configured properly and the softphone extension 5802 and i2004 7625 are in the context incoming1.
the nortel i2004 displays a message on its screen :
"Please enter a Terminal Number (TN) : " ?

Please let me get through this problem.