Asterisk to avaya cs1000e sip not working

I have a Avaya cs1000e (Nortel) which is connected to Asterisk via SIP trunk. Currently am able to make call from Asterisk to cs1000e extension but not back. When am dialing Asterisk extension from cs1000e am not getting any response. I have checked the log on freepabx and found that there is no request comming from cs1000e .But the eth 0 of MY ASTERISK machine shows log of the request . So what i fee is that the request from cs1000e is reaching my ASTERISK machine but not on to the free pabx. Need this to work very badly as am running on a dead line . Please help me .