Hi himdp,
When you call from outside and when you press zero for help , you are not diverted your call to the desired number (0) because the ext(0) is a central device it self, but if you dial direct ext in this case we are able to receive a call without any issue.
And also we cannot make an internal call from the central machine(no power), Just show on his screen (night Serv)
i have tried to restart board, and also removed a cable and connected again(also changed cable and device) ,but no luck.
i hope the explanation above gave required information to solve this issue.
Thanks