How do you miss calls? Turrets not ringing? Is it from a specific number, area code or exchange? It could be a turret setting. Do you have a DND key anywhere on the turret? If so and its red, hit the DND key to turn Do Not Disturb off. You may also want to check ringing; MENU>Ring>Ring> check the lamping and set accordingly>save>quit.
How is your turret system integrated with your Nortel system? QSIG? How is dial tone delivered to your Nortel? Sip?
The latest IPC version I would work with is 15.3.3, newer than that I have heard has issues.