The problem is definitely an incorrect configuration.
I created a sip trunk between an Asterisk and a snom one , the first have two trunks (one inbound and one outbound) type "SIP Gateway"
pointing to a single trunk sull'asterisk .
Calls from an extension registered on snom one, are rotated correctly toward the Asterisk, while the opposite does not happen.
On the snom one I get the following error
[8] 2013/12/20 12:33:26: Could not find a trunk (2 trunks)
[7] 2013/12/20 12:33:26: Set packet length to 20
[6] 2013/12/20 12:33:26: Call-leg 24: Sending RTP for 5d667fb07f40a34e5884ff734539d1d8@192.168.XXX.4:5060 to 192.168.XXX.4:13694, codec not set yet
[8] 2013/12/20 12:33:26: Incoming call: Request URI sip:601@192.168.YYY.4, To is <sip:601@192.168.YYY.4>
[5] 2013/12/20 12:33:26: Received incoming call without trunk information and user has not been found
[8] 2013/12/20 12:33:26: call port 24: state code from 0 to 404
[7] 2013/12/20 12:33:26: Set packet length to 20
[8] 2013/12/20 12:33:26: Hangup: Call 24 not found
[8] 2013/12/20 12:33:26: Clearing call port 24, SIP call id 5d667fb07f40a34e5884ff734539d1d8@192.168.XXX.4:5060
I have checked all of the trunk and outbound route of the Asterisk, without finding obvious problems.
Thank for the help