Flash the LED for alerts on iPhone
If you can’t hear the sounds that announce incoming calls and other alerts, iPhone can flash its LED (next to the camera lens on the back of iPhone). The LED flashes only if iPhone is locked.
Tip: LED Flash for Alerts is a useful feature for anyone who might miss the tones associated with calls and other alerts in a noisy environment.
Go to Settings > General > Accessibility > LED Flash for Alerts, then turn on LED Flash for Alerts.
To prevent LED flashes when iPhone is in silent mode, turn off Flash on Silent.