Message failed to send notification on iPhone boot

Hello all,

I encountered an iPhone bug, where, after sending a few SMS messages without network coverage, I would be prompted with notifications "Message failed to send" after every phone startup, even after deleting the chat with the contact.

To remove these notifications, you just have to do an iTunes backup on a computer, then a factory restore with another computer. After that, when the phone asked if I wanted to restore from a backup, I selected the option "from a PC" and connected back the computer with the backup. The backup was then restored and I the notifications "message failed to send" after each phone boot were gone. This is the solution!