While iPhone calls these days are vastly better than those of iPhone models in the past, let a lone calls made on those old flip phones, they can still be pretty cruddy, depending on your service coverage and location within that area.
If you’re at home (or at a place with WiFi) and in a bad location for cell tower reception but still need to make a voice call, you can do one thing to immediately improve the audio fidelity of your iPhone to iPhone calls: use FaceTime Audio.