Asterisk breaks connection after collect

I have this problem:
My Asterisk call by SIP account to my phone
when I collect my phone asterisk breaks the connection

in logs : SIP/sip.voipoperator.com-00000003 is circuit-busy

I have Asterisk behind a NAT

in sip.conf I add:
srvlookup=yes
dtmfmode=rfc2833
nat=yes
defaultexpiry=60

What could be the problem?