Wired
iPhone remains silent on iOS 11.2.5
The bug, which causes no sound and / or vibration to be played when a new SMS is received, affects a few users since the software update on iOS 11. While push notifications from other apps play a sound as usual, the iPhone remains mute for SMS alerts. Only a glance at the screen reveals the users currently, whether they have received an SMS or not. For iMessage notifications, however, the problem does not arise and some SMS messages from the respective mobile service provider are also received with sound on the affected iPhones.
While it's not known how many iPhone owners are really affected by the issue, the long thread in the official Apple forum suggests that a large number of iPhone users are suffering from the bug. Even iPads are affected by the problem. First, it was suspected that the cause of the bug was the pairing with an Apple Watch, since here too the SMS did not arrive directly. This turned out to be wrong, as devices without a paired Apple Watch also played no SMS notification sound.
Assumptions that it would be connected to connected Bluetooth devices or the "do not disturb" function have not yet been confirmed. Even the update to iOS 11.2.5 does not solve the problem. In some cases it would probably help to turn off the corresponding notification tone in the settings under Sounds & Haptics off and on again. Apple itself has not yet commented on it, so this issue just joins the long list of iOS 11 bugs. To avoid such errors on iOS 12, this version of iOS will probably focus on system improvements and bug fixes rather than including many new features.
Tip: Your iPhone is not just mute with SMS notifications? This can have several causes! We have put together all sorts of problems around the sound on the iPhone and their solutions here for you.
Source: iPhone Tricks