SIP and NAT

Hi FG admins

From this tech tip:

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Most-common-cases-of-SIP-implementation/ta-p/190676

I have this scenario (phones behind NAT):

Did all the required config, and even more:

config system settings
 set sip-expectation disable
 set sip-nat-trace disable
 set default-voip-alg-mode kernel-helper-based
end

config system session-helper
 delete 13
end

config voip profile
 edit "default"
  config sip
   set rtp disable
   set contact-fixup disable
  end
 end

...

Played with the above parameters and FG reboot but didn't work. I mean I have this behavior:

• Calling from internal IP phone to external mobile cell phone (GSM): It rings but no voice
• Calling from internal IP phone to internal IP phone: IP phone doesn't even ring

In the traffic logs I could see some "TCP reset from server" on SIP connections.

So I'm starting to think that probably on server side must be somehow configured to accept calls from IP phones behind NAT. Anyone knows something about that?