Hello there jimhabermehl!
Your phone has several microphones that it uses to pick up sound and send it to the folks on the other end of the line. Do you have a phone case? A common issue is that a phone case can obstruct the microphone and make it hard for sound to get through the case. Another possibility is the microphone could be dirty. Maybe try spraying the bottom of the phone (near the lightning connector) with compressed air to try and free some of that dust from the device. It is also said that updating your iOS can help in certain instances.
Let us know if that helps solve the issue! Our awesome community will probably also provide some good insight.