The problem with the questions and answers is that people interchangeably use the term "DDNS Hostname" and "DDNS IP Address".
If someone wants to use DDNS, they put the DDNS Hostname in the External Address box. Do not put in the current external IP address. The system checks every 15 minutes to see if the DDNS Address has changed and the system makes the appropriate adjustments.
The second problem is that (historically) the external address is supplied by the browser and not the actual server, so if you connect from outside the network (like I do from my management console at my office) and supply the IP address that gets reported, it will be wrong. The address supplied is the external address on my office network and not the address of the server. Important note - this behavior may have changed or may change at any moment. The point is to make sure the external address you are using for NAT is the correct address. If it changes or you don't know the address, don't guess - you have to get this right.
IMHO - there is no excuse for setting up a PBX (especially with inbound traffic) on a system that doesn't get a dedicated IP address. I understand there are teenagers here that don't have the resources to do this right, but there's been a lot of effort expended on this system for things that I personally shouldn't be as high a priority as it's been.