Check Your Phone Case
One of the most common problems afflicting iPhone calls is a poorly positioned case. There are innumerable cases out there, and not all of them were made with the iPhone’s mic and speakers in mind. This is especially true of the bulkier cases that add extra features (wallet pockets, water-proofing, batteries, etc.). So if you notice you’re having troubling hearing or being heard, please check your case. Take it off, see where your speakers and mic are, and note if your case is blocking them.
For newer iPhone models without a Home button, the main mic and speakers are located at the top of the phone, close to the camera and at the bottom of the phone, by the charging port. If the case overhangs these areas too much (especially the bottom section), it could easily interfere with communication.