Remove all modules except one station module and trunk module. With all other modules removed (of course you can't remove BCC, MCC) make test calls. See if the condition persists. With one station module, you can check if station to station calls drop. If all tests well, start adding modules until you get a failure.
I have had this problem and it was a bad BONS card. It could also happen to a regular station or trunk module. All the Mitel modules have common logic controls on them and if one turns sour, it can bring down the whole system.
Rcaman
