SIP Trunk between Nortel Opt. 81 C and Asterisk

HIIIIIIIII friends

i have Nortel Opt. 81 C and Asterisk. i want to establish a SIP trunk between them
i can call from Asterisk to Nortel.
but i can’t call from Nortel to Asterisk

the call go out from Nortel but there is some thing i don’t know prevent the call to reach to Asterisk

(calldate,clid,src,dst,dcontext,channel,dstchannel,lastapp,lastdata,duration,billsec,disposition,amaflags,accountcode,uniqueid) VALUES ('2008-08-09 05:09:33','\"HUSSAM AL-HAMARSHEH\" <1566>','1566','6700','from-sip-external', 'SIP/172.16.35.21-0844a2e0','','Congestion','',0,0,'NO ANSWER',3,'','1218272973.7')
Aug 9 05:09:33 DEBUG[3214] chan_sip.c: update_call_counter() - decrement call limit counter
Aug 9 05:09:33 DEBUG[3214] chan_sip.c: update_call_counter() - decrement call limit counter