The issue here is that your carrier is detecting you as a NATed client
and this breaks your ACK routing.
I overloaded the meaning of interfaces. Two sockets (same IP,
different ports) works fine in a multihomed setting.
One socket for internal traffic and one socket (with external
advertised IP)
It seems that you SIP provider detects your NAT settings and it is
trying to fix it, which breaks your setup.
See the Contact in the 200OK: Contact:
sip:0390156...@kamailio.int:5070;transport=udp
Which is bad, it should be the IP of the FS server.
See the RURI in ACK:
