Check for silent mode once in iPhone, maybe accidentally put it in. If it is then iPhone not ringing is solved by removing it.
Simply, operate the volume buttons of iPhone to raise and lower the volume levels and to inspect the levels are correct or not.
1. Open iPhone and navigate to the “Settings.” 2. Next, open the settings and look for “Sounds & Haptics.” 3. Now slide the slider up, keeping the maximum ringer volume.
DND can be turned on accidentally on iPhone and it can induce troubles if you don’t realize you turned it on.
Proceed to Settings -> Silent Unknown Caller and inspect which numbers are fetching blocked. Release the numbers which you don't like to silent.
Fix the issue "iPhone not ringing" will be attempting to restart the iPhone and scan if it produced any difference.