I was digging with wireshark trying to understand what's different.
It's better to start from the begining.
Let me try to share what I found and start the discussion from the pure networking point of view.
On ISSABEL I've set NAT=YES, Dynamic IP, and configured my localnetworks.
With this, when I do a sip show registry, I get a 1365 refresh time and I'm NOT able to receive INBOUND calls.
Register from ZOIPER to my provider:
Via: SIP/2.0/UDP 188.83.8.35:55894;branch=z9hG4bK-524287-1---4946bc687144761d;rport
Source Port: 58150
Dest Port: 5070
Register from ISSABEL to my provider:
Via: SIP/2.0/UDP 188.83.8.35:5060;branch=z9hG4bK74dcef74;rport
To start, as you can see zoiper sets a random port 55894. ISSABEL always do 5060. I do NOT have port forwarding configured to point 5060 to my ISSABEL box.
If I set ISSABEL as IP Configuration = Public I get this Register:
Via: SIP/2.0/UDP 192.168.103.11:5060;branch=z9hG4bK24c4295e;rport
192.168.103.11 is my issabel box.
I get a 105 refresh time when I do sip show registry and INBOUND calls work.
Lets start from here, with the above scenario why does Zoiper receive inbound calls and issabel don't? Any hint?