We're starting to get into pretty unlikely territory now, but it's worth investigating your ringtone to make sure that the file isn't corrupted, or the ringtone itself isn't just silence.
Why would it be silent? Well, one tactic that some people use to deal with torrents of telemarketing calls is to configure their phone with a ringtone that's literally just several seconds of silence for all phone calls except for friends and family.
1. Open the Settings app.
2. Tap "Sounds & Haptics."
3. Tap "Ringtone."
4. Tap the ringtone that's currently selected (with the checkmark) to make sure it plays. Or just select a different ringtone.