It's FreeSWITCH trying to automatically detect the ${local_ipv4} and ${local_ipv6} values.
It does this by attempting to send a packet to an IP it knows will have to go over the Internet and seeing which local IP the packet would leave the machine from. That means if you have multiple IPs assigned to your machine it'll know which one will send via the default gateway. That handles the simple single-homed case anyway.
It doesn't really matter whether NAT happens or not, it works just as well if your machine is listening on the public IP natively.
It's just trying to auto-detect what IP you're most likely to be reachable on.
Try a packet trace... from memory it only creates the socket, doesn't actually send the packet (?). But I may be wrong there.
-Steve