If the sounds that announce incoming calls and other alerts are hard to hear, the iPhone can flash its LED (next to the camera lens on the back of the iPhone).
Follow this step by step guide to enable LED alerts.
From the home screen, tap Settings.
Scroll to and tap General.
Scroll to and tap Accessibility.
Scroll to LED Flash for Alerts, then tap and slide the toggle to the right to enable the feature.
LED Alerts are now enabled.