Double NAT is a term used to describe a situation when a phone is behind two routers in line, and they are both trying to route the traffic. The router is the boundary between the public and private subnet. There can only be one router to ensure voice quality.
This can be caused by having a router/firewall behind an ISP gateway that has not been properly bridged. Calling the ISP and asking them to bridge the gateway usually fixes the problem.
Some ISPs cannot bridge their gateways due to the structure of their service. There are still some settings that can be adjusted.
Feel free to contact the tech support at firstname.lastname@example.org if needed.