asterisk users - Nov 2014 - Configure Asterisk as SIP UA using NAT

Hi

I have installed Asterisk 11.13.1 on Fedora running in VirtualBox. The VB
network interface is configured to use NAT. The host machine is Windows 7 and is
connected to a SIP server using a VPN connection.

I have configured ?externaddr?, ?localnet? and ?nat=force_rport,comedia?.
Asterisk registration is successful, I see in Wireshark the packets send between
Asterisk and SIP server.
However, when I try to call using a softphone, installed directly on the Windows
host, I see no incoming packets from the SIP server to Asterisk, so it looks
like my UA is not accessible from the SIP server.

Could you tell me what I am doing wrong ?

regards
Bogdan