Check your sound settings
If your device has a Ring/Silent switch, move the switch forward—toward the device's display—so that orange isn't showing. If you're using an iPad, you can also swipe down from the top-right corner to view Control Center and make sure that Silent Mode is off.
Open Settings > Do Not Disturb and make sure that Do Not Disturb is off.
Clean your receivers and speakers
Remove all screen protectors, films, or cases from your device. Check the speaker opening to see if it's blocked or dirty. On iPhone only, make sure that the receiver is not blocked or dirty. If necessary, clean the speaker or receiver opening with a small, soft-bristled brush. Make sure that the brush is clean and dry.
Check the sound on your device
Go to Settings > Sounds (or Settings > Sounds & Haptics), and drag the Ringer and Alerts slider back and forth a few times. If you don't hear any sound, or if your speaker button on the Ringer and Alerts slider is dimmed, your speaker might need service. Contact Apple Support for iPhone, iPad, or iPod touch.
Try making a call
If you hear sound, try making a call with your iPhone and turn on speakerphone. On iPad or iPod touch, make a FaceTime call. If you still can't hear, or hear static or crackling, then your network or reception could be the issue. Try to call again later, or from a different location.