NAT handling

Hi Experts, I worked for years with Cisco and Ingate SBCs. Recently we have deployed an Acme 3820.
I've a doubt about 2 scenarios and probably you can clarify.

 

- CASE 1

 

EndPoint --- NAT --- SBC --- PBX

 

In the manual I see this note about NAT:
nat-traversal always (our actual config): SIP HNT applies to requests when the sent-by of the topmost VIA matches the Contact-URI host address, both of which must be different from the received Layer 3 address. (Even when the rport parameter is not present.)

 

If I've correctly understood, the Acme recognizes a NAT only if the VIA IP is equal to Contact IP and both are different from Layer3 IP of incoming packets:      NAT = (VIA AND CONTACT) NOT LIKE L3 IP