Another thing to think about is that this is an old system.

This failure might be the right time to go to the owner and say, "We've gotten our money's worth out of this Mitel, but we need to replace it now."

SX-50s are still in service, which is a credit to Mitel. But there comes a time to move to a newer system. You would love the upgrades and features in a Mitel SX-200 EL, or a Mitel SX-200 ICP.