I will get the log files as soon as possible.
The wall socket isn't the problem, I already ruled that out.
When I add a specific extension into the main ring group then I can phone in, but as soon as I remove that extension it says call failed.
What might be the problem is that my supplier probably configured the routing incorrectly. When I dial the number it is suppose to go to the queue which is 8000 for instance. The inbound rule specifies that, but instead it goes to extension 1641 which doesn't have an inbound rule.