Apple’s iOS 8.0.1 has introduced a nasty bug that breaks cell phone service and Touch ID. From most reports, the issue is specific to the iPhone 6 and iPhone 6 Plus. You have a few options here, none of which are great, but can help you get your cell service back.
Another option is to restore your iPhone to iOS 8.0 using iTunes. Apple has already pulled iOS 8.0.1. When you go through this process, iTunes will download the latest working version of iOS. This seems to be the easiest and most effective way at fixing cell service on your phone. It also means that you must backup all of your data.
1. Connect your iOS device to your Mac or PC. Open iTunes if it does not launch by itself.
2. Select your iPhone either from the left hand menu or from the top right.
3. Under ‘Summary’, click on ‘Restore iPhone…’
4. You’ll be asked if you are sure you would like to restore your iPhone to its factory settings. Again, you’ll receive a warning that all of your media and other data will be erased. Select ‘Restore’ to continue.
5. iTunes will download the latest version of iOS 8 and start the installation process. During this, your iPhone will reboot. You should see a black Apple logo against a gray background. Below it will be a progress bar for the software install.
6. Next you’ll see a message pop-up in iTunes that “Your iPhone has been restored to factory settings, and is restarting. Please keep your iPhone connected. It will appear in the iTunes window after it starts. Select ‘Restore from backup’.
Finally, Apple is well aware of the issue. It would be hard to imagine a fix not coming very soon. If you need your phone working, this is the quickest way to get it back up and running.