iPhone says ‘0 bytes free’ no matter what I remove

You should check your Usage for messages. I've seen people have GB's of text messages, and since the phone doesn't automatically delete messages (pre iOS 8), they'd just sit on the phone.

If your device is full, the update won't take. Your best bet, if you can't figure out whats taking up the space, assuming you've gone through your Usage and can't find anything else to delete, is to restore the phone. It will delete everything, but if there is some sort of corruption, it may be the only way. If you've already deleted all your stuff off the phone anyway, it won't make much of a difference.

Hello FifoFolta,

Thanks for contributing to the Apple Support Communities.

It sounds like your iPhone is reporting "0 bytes free" when you check the available storage under General > Usage, even after removing some data.

Since this happened when you updated to iOS 8.3, I recommend using these steps to reinstall the iOS software:

1. Connect your device to a computer and open iTunes. If you don't have a computer, you can borrow one, or go to an Apple Retail Store or Apple Authorized Service Provider for help.
   
2. While your device is connected, force it to restart. Press and hold the Sleep/Wake and Home buttons at the same time. Don't release when you see the Apple logo. Keep holding until you see the Recovery mode screen.
   
3. When you get the option to restore or update, choose Update. iTunes will try to reinstall iOS without erasing your data.

iTunes will download the software for your device. If it takes more than 15 mintues, your device will exit recovery mode and you'll need to repeat steps 2 and 3 again.

From:

If your iPhone, iPad, or iPod touch doesn't respond or doesn't turn on - Apple Support

Although data shouldn't be erased, it's always a good idea to have a current backup before troubleshooting:

How to back up your devices using iCloud or iTunes - Apple Support

Best Regards,

Jeremy