Ringer not working

Some things to check:

The ring/silent switch on the left side of the phone. See the following Apple article: Use the side button, Home button, and other controls on your iPhone - Apple Support

Check the ringer volume at Settings > Sounds & Haptics > Ringtone & Alert Volume.

Settings > Phone > Silence Unknown Callers. See: Detect and block spam phone calls - Apple Support

Do Not Disturb. See: How to turn Do Not Disturb on or off on your iPhone - Apple Support

If none of the above, also try restarting. See instructions in: Restart your iPhone - Apple Support