European phone can't register to AT&T: Error GA205

That's probably the reason then. 

The model number does make a difference. His model is not on the list, hence not supported.

OK,
so he should change provider or phone?

I've chatted with T-Mobile and they check via his IMEI number and according to the, the phone is compatible.

Should I trust them?

That's up to you. If you are happy with AT&T, get a new phone. If not, and T-Mo says his phone will work, then switch to them.

Since he's only in the US for 4 days, we don't have any affinity with either provider yet ;-)
Thanks you very much for your assistance!

I still wonder why it worked once

It will work on T-Mobile. The issue with ATT is they now only allow phones that have gone through ATT’s certification process due to the pending shutdown of 3G. Manufacturer’s don’t normally certify their international models.

I've rarely seen such (Edited as per community guidelines) on a forum as these responses.

My phone connects and works flawlessly on AT&T with my foreign sim, but when I try to register the pre paid.sim, it fails with the (Edited per community guidelines) ga205 error.

So they dare not risk losing international roaming $$$, and do this merely to discourage you from using a local sim, and at the very least force you to an unnecessary phone purchase.

Well I hard selected my network to T-Mobile in my dual SIM phone so they'll get nothing further from me in these next two months and your sim goes to the 'round filing cabinet'.

Dr. Un Happy

Hi

Same issue for my french daughter who travel in USA with a sim card from FREE.

She is panic as not possible to join anybody ! No network at all ! It's a pity !

Her phone is a samsung A50 LTE/4G she has since two or three years (don't remember exactly)

How can we figure out this issue please ! it's urgent for her to be reachable !!!

Hi Ludroy,

Get a T-Mobile SIM card, that worked for my son and my wife.