UDP works only with username and password, not with the MAC address. Firewall, I can register any other phone behind this firewall, just not the new batch of phones we recently ordered. The phone, base, simply doesn't react to the address of the domain weather is https://mydomain.com or http://mydomain.com. The certificate on the server is current. Unfortunately Yealink doesn't have any logs we can look at to determine what is wrong.