I first noticed that my iPhone wasn't ringing for incoming calls. Even worse than missing calls, the Phone app kept freezing up; I couldn't navigate between contacts, the dialpad and voicemail. After forcing the Phone app closed a few times, the behavior continued. But that wasn't the worst of it.
For several days after the iOS 8.4 upgrade, my iPhone 6 was running hot. Not just warm but hot to the touch; particularly at the top right of the back of the phone and along the right edge. The battery was draining much faster as well; likely coinciding with the sudden heat. Even when in an idle, unused mode, the battery would drop from 100 percent to 50 percent in just a few hours.
I tried a few fixes ranging from resetting the phone's settings and forced reboots -- holding down the home and power button for about 10 seconds -- but nothing worked. In the end, I went with what I call the "nuclear" option: Wiping the phone and reinstalling iOS from scratch.
Why go this route? I figured the software update had caused something buggy at the system level. There were no new apps installed after the iOS 8.4 update and none of my apps were upgraded either. The likelihood of a third-party app as the root cause seemed very slim. Since the native Phone app was wonky, I felt comfortable with that assessment.
It's actually not difficult to do the full 1.6 GB installation of iOS 8.4 on an iPhone, but you'll need a device running iTunes to do it. And you'll have to put your iPhone in recovery mode as well.
To start, turn off your iPhone and connect it to a computer that has iTunes installed while holding the handset's home button. Then simply keep holding the home button of the iPhone for 10 seconds: iTunes should then notify you that it found a phone in recovery mode.
Choose the recovery option in iTunes and it will download the full iOS 8.4 software; not just the incremental changes found in the update.
Once my installation completed, I chose to set up the handset as a new iPhone; more on that in a minute.
The end result after using the recovered iPhone was a normal battery drain, a working Phone app and no excessive heat from the handset. For testing purposes, I only installed Twitter and Facebook after that. The next day, I began to reinstall other third-party apps and haven't had a problem since.
Clearly, this method removes all data, documents, music, videos, photos and apps. So you might want to make sure you have a backup of your iPhone before you even start the recovery process.
I personally don't ever install a backup on any of my phones or tablets though; instead, I set them up as new devices. Why not? That's another story for another time, so later this week I'll explain my reasoning.