Has anyone had this problem with the S Class 56s? When people are on the front desk phone or phones with other customers, say lines 1 and 2 and another call comes in on line 4 you hear a slight crackle and just the front desk phones that people are on calls with reset. I tried moving the co to a different port and different card, same affect. Even changed out the cabinet with same software level 4.07.18 and it does the same thing. Replaced the power supply, changed phones no change. Took voltage readings on all 7 lines and range from 70.3 to 73.4 vac. Seems low. Any ideas?