Open Control Center
Tap the bell icon to turn off Silent Mode
If the bell icon is not visible, use the Ring/Silent switch on the side of the iPhone
Move the switch so the orange color is not showing
Go to Settings
Tap Sounds & Haptics
Under Ringer and Alerts, increase the volume
Make sure Do Not Disturb or Focus mode is turned off
Check that Bluetooth is not sending audio to another device
Test by calling your iPhone