Apple will fix the bug in the upcoming iOS 11.3 update.
Crashes, freezes and other issues will appear as soon as someone sends you a simple text message containing a specific Indian character. This bug has been found in the current official versions of iOS – 11.2.5, and macOS – 10.13.3. A specific letter in Telugu sent via the built-in messaging app, or some third-party chat clients like WhatsApp and Facebook Messenger, will definitely crash your iPhone, unless you are running the older iOS 10 or the recent public beta of iOS 11.3. This means, the issue currently affects a large number of devices, but an upcoming official update will soon fix it for everyone affected.
If the problematic character is sent using the iMessage app, it freezes the app and makes it unusable. Ever worse is the fact that the app refuses to work even after you restart the phone, or until the letter is removed by the person who sent it.
In another scenario, if the indian character appears in the notification banner, the whole device crashes. If you delete the conversation in the app, the problem will be resolved. However, this is pretty annoying bug in iOS, which hopefully will be fixed in a matter of a few weeks.