Rep erroneously said iPhone was modified, said to go to Apple to fix
I had just upgraded the main phone on my account to an iPhone 8 Plus and passed the 6 Plus to my father, who had been on a non-data line.
I had to visit a retail location, chose nearby Amsterdam, New York, for the 6 Plus. They issued a new SIM card and in short order, was successful and I then transferred information to that phone. However, the Amsterdam people didn't look at my account deeply enough and never changed the line assigned to the 6 Plus to be on the shared data plan. We didn't realize that was needed because at the time, the WiFi at the store sufficed.
Two weeks later, after repeated tries, I suggested they go back to Amsterdam. The rep there saw that the 6 Plus had a screen with a white border (which I had professionally replaced after breakage). The original glass had a black border, but the white was the only glass available to the repairer, and was very capable and acceptable. Still, when the rep saw that white border, he then declared the phone to have been modified and to see Apple, rather than to make the appropriate changes to the account. I had given the account passcode to my parents, so they could request this change. At no time did the store attempt to contact me for clarification or authorization, they simply refused to make the change.
A day later, using my own phone, from my parents' home (an hour's drive for me) using chat in the AT&T iPhone app, I was able to get the account modified to put the 6 Plus onto a data plan with virtually no effort other than to supply the IMEI and other numbers necessary.
Please educate your retailers that a phone that may appear to have been modified may have not been, and that they need to give things a full look rather than a snap judgement to refuse service.