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

Without checking the supported phone list, I’m pretty sure the problem is his phone is not on ATT’s supported phone list and ATT will no longer activate a phone not on the supported list. 

It could be that the IMSI number for a non-domestic phone is not recognized on the AT&T network. It is almost impossible for any carrier to recognize every single IMSI number for all carriers world-wide. The IMSI number is comprised of the MCC, MNC, and the MSIN identity codes.

If that is the problem then I don't think AT&T can change it because I believe the IMSI number may be embedded on Android phones and not on the SIM card. He may just need a new SIM card tho.

Hi,

thanks for your answer.

Isn't it strange that it worked once in the store?

I'd think this would be a binary problem: either it never works or it always works?

Furthermore, his European SIM card is working on the AT&T network

Did he get a new SIM? It is strange that it worked in the store but not out in the wild. Has he reset the phone? Maybe that's what it needs as well as a new SIM.

I missed that it was a prepaid phone. I don't know how that would affect a European Android phone but we do have an Android expert here so maybe he can shed some light.

He got a new SIM yesterday, but he didn't activate/try it in the store.
When he was "in the wild" and tried to activate it, he got the message "GA205: ... We're sorry, but the phone you recently tried to activate won't work. ..."
So he went to an AT&T shop today and they helped him to activate it, and the shop assistant was able to call him on the new SIM/number.
But when he was back "in the wild" he received the same text message again

Don't they check this when he is in an AT&T shop specifically for this problem?

They should.

Samsung Galaxy S8 is on the supported phone list, but he has the G950F and on the list is G950U
I don't know if that makes a difference