[Sony Xperia XZ Premium Questions & Answers] GreenDev9:
I have a G8142 with the latest security patches/updates on AT&T.

I'm having a strange issue occur on probably one out of every two or three calls. After about 45-60 seconds of conversation, my mic will cut out. I can still hear the other person clearly. I can end the call (this is not instant - it hangs up for 3 to 5 seconds) and then call the person back and resume conversation normally.

A few notes:

It doesn't matter if I place the call or if I am called

The duration from beginning of call to cutting out fluctuates

My microphone diagnostic indicates it is working properly

The call does not seem to actually drop - it will go on for quite some time (with me able to hear the other person) until someone eventually hangs up

Calling the person back/being called back does not fix the issue (it can happen multiple times in a row with the same person/call sequence)

Signal strength is usually -80 or better

The phone functions flawlessly otherwise. There also don't seem to be any issues or drops with respect to the data connection, and SMS/MMS messages seem to get delivered immediately every time. The phone hasn't been dropped or exposed to any liquid. I have disabled some system apps (like Facebook), but none that seemingly have to do with making phone calls.

I have full bars most of the time. I force 3G+ on the phone, as the 4G+ (LTE) signal is weak in many areas. AFAIK that only affects data rates anyway.

It doesn't seem to happen as much if at all if I put the call in speaker mode. I have not used a bluetooth headset for calls to see if that fixes the issue.

Here's some things I've tried:

Rebooted the phone

Updated my IMEI with AT&T. I'm not sure if they actually did it (they stated it was "already changed") or if it makes a difference. My last phone was an unlocked Nexus 6p and did not have this issue. I did not get a new SIM for this phone.

Double-checked and redownloaded the APN settings.

I haven't tried voice calling through a third-party app, like hangouts or whatsapp, but I assume it would work just fine because it's being transmitted over data.

There is either a GSM/setting issue on my end, a settings issue on AT&T's end, or possibly a hardware issue (like a loose SIM, faulty mic, or bad antenna).

For now I'm at a loss, and any ideas would be greatly appreciated.

*** UPDATE ***

I've tried clearing the phone app cache and making sure that battery optimization is turned off for the app. I'll report back if this seems to fix it in case anyone is having a similar problem.

*** UPDATE 2 ***

That didn't work. Calls still drop intermittently. I've pretty much narrowed it down to a problem with the antenna, AT&T towers not particularly caring for the baseband firmware on this phone, or an issue with the dialer app. Performing calls over data (like Whatsapp/Hangouts/Duo) works flawlessly.

We'll see what happens if the phone app gets an update, or maybe after Sony releases the Oreo update.

For those US XZP users on AT&T or those that were considering buying this model, be aware this issue may affect you.

*** UPDATE 3 ***

Changing the mobile radio to 4G (preferred) instead of forcing 3G seems to have solved the problem.