I think the microphone (or related) has gone wonky

I've been having serious difficulty communicating to my iPhone from the car lately. The iPhone is connected to the car with an AUX input and Lightning/USB cable. I've been troubleshooting the phone and it seems to be working properly, so I suspect the car's microphone or whatever module follows that. Partly because I don't know much, I haven't found much searching this site. Seeking any input on what parts I should consider replacing, and where they are (headliner?). Many thanks.