I performed a tcpdump on my internal firewall, Fortigate 311B, (the SX20 is on another network, routed through the Internet) and did not detect any record of SYN ACK on the firewall, towards the VCS-E for the SX20.
Then this same check performed on another endpoint, also an SX-20 in another network under the same conditions.
I've found that the same problem occurs TCP checksum error, but the same internal firewall SYN ACK packet is forwarded in the sense VCS-E for the SX20.This equipment is typically logs in VCS-E (SIP port: 5061 transport: TLS).
I performed another check in VCS-E in relation to its NIC, referring the "checksum offloading" was enabled:
I accessed the VCS-E via an SSH connection, and gave the following command:
ethtool - show-offload eth0
In response I got:
Offload parameters for eth0:
For the answer above is not the problem this variable, I have no idea what could cause this error, since I have another VCS-E and performed the same tests and got the same results.