My advice to you would be, read about, and understand the SIP protocol, there are lots of resources out there on the internet. Also make sure you have an understanding of IP networking. Once you have this understanding, you will be able to set up packet capture that will allow you to see what is happening between your FusionPBX installation and your provider.
I assume your FusionPBX is trying to Register with your provider. You can read about the SIP registration process here:
https://andrewjprokop.wordpress.com/2014/05/29/understanding-sip-registration/
What I have said may not sound very helpful to you right now, but believe me, it will help you in more ways that you can imagine in the future, especially if you are thinking of providing Fusion to customers.