We have just recently taken over an account that the previous vendor had a Comdial MP5000 installed with 1 expansion cabinet. This cabinet was damaged beyond use by a lightning/power surge. The vendor offered to replace the system with new NEC equipment or replacing the main cabinet and expansion cabinet with used/refurbished equipment and leaving the phones the same. The client (because of budget) chose the used/refurb equipment. Since they have installed the used equipment they cannot have more than two IST cards mounted. If more than two then they receive a static popping when calling into any DID number(they are on a PRI) We have swapped cards to try to pinpoint if a backplane issue or card issue. The current active cards are slot 1-PRI, slot 2-7 digital cards, slot 8-10 IST, slot 11-digital, and slot 12 IST. We currently have slots 10 and 12 unseated because of the static popping. When we unseat any two IST there is no popping. I can provide more information if anyone willing to reply with any help. Thank you in advance.