I have configured a Sipura 3000 for FXO and FXS which appears to work well, except for an odd sound problem. Sometimes when the phone is off the hook the sound will fluctuate sometimes to the point where it cannot be heard. I am assuming that it is a line voltage problem or a bad box, but I have not been able to find anything to troubleshoot the problem. I have seen adjustments to gain etc, but not for the spa3000. In the US (Texas). Any help would be appreciated. Thanks; Dan