Symptom: You can barely hear the audio when making a phone call but the quality is fine when using the speaker phone. Turning the volume all the way up has no effect.
Fix: iOS during an update likely enabled the noise cancellation setting. Although this can be useful, it has a habit of making phone calls very difficult to hear when you use the phone with it up to your ear. Disabling this setting immediately returns phone call volume back to normal. This setting is located at:
Settings –> Accessibility Phone > Noise Cancellation