Dear Sardar Muhammad Nurul Huda
My first guess would be that you have an A800 and you are not using the splitter Y cable that comes with the unit. The first RJ45 port on the PSTN side is actually lines 1 and 5, port 2 is lines 2 and 6, 3 is lines 3 and 7, and so on….
You need to use the Y cable, plug the single end into the RJ45 port – you then have the cable marked A and B at the other end. A is line 1, B is line 2.
You cannot use a standard straight through cable as it will not work correctly and probably give you the sort of errors you are getting.
If you need any further assistance please feel free to contact me.
Hope this helps