Are you connect to a Mivoice Connect or MiCloud Connect system? We sell the Cloud product and that is normal behavior, but the app will launch when running the background and/or when the phone is locked.
We see 2 sets of behavior. Usually the app will pop but sometimes if the data stream is delayed, the Iphone processes the call in what looks like a native IOS call screen, but the contacts button is replaced with a Mitel logo. Hard to explain but you will know it when you see it. You can then answer the call and f you want to populate to the App, you hit that button,.