gives a messge "error has return" for incoming callers

Hi All

I have installed Trixbox pro as my IPPBX. I am using X-Lite softphone and sipura spa 3000 device for pstn line in to the IPPBX system. I can talk within a office, receive incoming calls and can take out going calls. But some time suddenly it gives a messge "error has return" for incoming callers and then next minutes it connecting to the system. this error occur when the line is free. any one can help me on this issue.

Thanks,

Wijendra.