Audio Echo During Phone Calls On iPhone - Head Scratcher

Hello All,

I have an odd one I haven't been able to crack for a customer of mine. Just recently they started getting their own voice echoing back at high volume during phone calls. This happens whether speaker phone is on or off. It sounds like the microphone is feeding it's audio directly back into the speaker. It is literally like talking to yourself on the phone, a conversation with yourself! The caller on the receiving end gets extremely low volume or no volume at all. The only progress I made was enabling Voice Isolation mode which finally got proper audio and volume to the caller on the other end; but the echoing feedback at a loud volume remains on the iPhone making the call. It's unusable making a phone call.

Model: iPhone 13 Pro 1TB version - Internal Storage is only using around 100GB, 900GB are free.

List of things I have tried:

Multiple Hard Resets

Multiple Resets through Settings - General - Shut Down

Wi-Fi - Both On and Off

Wi-Fi Calling - Both On and Off

Bluetooth - Both On and Off

Different Phone settings via Control Center - Automatic, Voice Isolation, Standard

Various Audio settings in Accessibility - Both On and Off

Headphones Accomodations - Both On and Off

Background Sounds - Both On and Off

Reset Network Settings

Took the phone out of the case

Cleaned entire phone with isopropyl alcohol making sure the microphone section was clean and nothing covered by the screen protector

I then took it to the Apple Store and their diagnosis came back good for both hardware and software. They said the microphone and speaker passed their testing. They thought the issue pointed to the carrier, AT&T. I took it to AT&T and they replaced the SIM card to no avail. I then did an iCloud backup and factory reset the iPhone and as soon as the phone started up and was able to make calls I tested it again. The same echo issue prevailed. I am at a loss on this one. The customer's warranty just expired this year and they are not happy about the lack of progress I have made on this. The most confusing part is that phone calls work perfectly with Airpods Max headphones via Bluetooth. There is no echo when using these and the caller on the receiving end gets perfect audio. Then switching back to the iPhone's onboard audio during a phone call the echoing persists. I also tried making calls in other parts of town on either 5G or 4G LTE and the issue persists. I'm suspecting a hardware issue like a bad IC on the logic board but there have been no crackling or static type noises to make that the more obvious culprit. Any ideas?