Nortel SIP phone problem

Hi! i have a nortel 1120E with the SIP formware phone. i can configure it normaly, i can make outgoing call with it. I must put canreinvite to no if i want it to work, it is not behind a nat or a firewall. the problem i got is as soon as i try to call this extension, it got a busy message like if the phone was already on a line. but it is not. What can make this problem? I have this message when my nortel phone long in the asterisk server:

Got SIP response 305 “Use Proxy” back from

If someone have an idea of what can be the problem, it would be of a great help thanx!

NO one have an idea of what can cause this problem? the always busy problem is it because of the Proxy problem ?