Is AT&T Flagging and Disabling SIM for Specific Phones in The US for use on their 5G Network

I always buy factory unlocked phones for use on GSM networks since I travel and plan to travel outside of the US. And I always select phones which are unique and unpopular. 

Anyways, I was informed, in the summer of 2021, that my phone would no longer work on the AT&T network because of compatibility issues when AT&Ts 5G rolls out in 2022. I usually keep my phones for 5 years just like my computers. AT&T offered to send me a phone that was not even comparable to the phone I purchases with advanced specifications, dual sim and the ability to add additional storage up to 256GB. 

I talked to the Customer Service rep and when discussed the phone I have currently with the phone that AT&T wanted to send me. It was noted that the replacement phone was not comparable. I kept the with the phone with the hopes that there maybe a firmware available by the time 5G rolls out. 

Phone was still working until April 17th with a result of SIM unregistered error. 

First issue I had was in March when traveling to JFK Airport on my way to France. Phone stopped working about a half hour away from the airport. Great. I then flew to France where my phone worked effortlessly in CDG airport. All over France, even in the metro. I have the international plan as an addon to ensure that I have service when traveling. 

Traveling back from France, the same issue. Though when I was about an hour away the phone connected back to the AT&T Network. This is good.

Then I travel from BDL to PHL (Philadelphia International), and the phone fails in Philly. I had to buy a phone because I was stranded the first day without access and that new phone, I got from Best Buy, because the AT&T rep in the store said they had none cheap to be used temporarily.  I paid $159 for the phone, Moto G Pure, which is supposed to be compatible with AT&T, but the Best Buy rep was unable to activate the SIM. 

I return back to AT&T. It takes a while. I get another SIM, eventually after some finagling it works. I get back to my hotel switch the SIMs phone. I works for like 3 minutes then fails. I put it back in the Moto G Pure. Doesn’t recover. I go to another AT&T store, I’m there for more than an hour and 2 SIMs later and the phone is still failing. I then check out T-Mobile’s international plans and explain my issue with the phone I want to keep. They perform a test where they put a T-Mobile SIM into the phone and place a call. And it works. It looks like T-Mobile might be an option.

It looks like BYOD might be an issue when connecting to the AT&T network. 

The list of phones that AT&T provided, that will work after the transition were existing phones and appear to be even model specific. 

It appears to me that there was a handshake and a verification between the phone and the SIM card. So I was unable to move the SIM between phones.

After going to AT&T multiple times (5) and T-mobile twice and being without a phone for 4 days, I called AT&T tech support. They were able to refresh the SIM with the phone to get it working again and this took about 15 minutes with a reboot. 

At one of the visits, I asked if my account or SIM could be flagged, and it was a definitive, no.

I am now searching for a new replacement phone not branded, to be used on GSM networks, and have the question:

Is AT&T flagging and unregistering SIM for phones, make and models not on their whitelist, using their 5G Network?