Hi Anagen,
We see you're getting some issues with your audio while on calls, and we'd like to see if we can help out there.
Trying out different microphones there was a great step to take (AirPods/speakerphone). Have you been able to try testing this out while doing a FaceTime audio call? In particular, try the FaceTime audio call while on a strong Wi-Fi network. This would help to rule out issues with the carrier's network on the device. If unfamiliar with FaceTime audio calls, it'll be the same as setting up a standard FaceTime call, but with just selecting the audio/phone icon instead.
If needed, this page can help with that part: Use FaceTime with your iPhone, iPad, or iPod touch
If the FaceTime audio call has the same exact issue, we'd advise reaching out directly to Apple Support for further investigation: Get Support
If it doesn't persist on FaceTime, just let us know, and we'll explore it further with you.
Cheers.