Could be the cellular radio used in the phone to locked to a carrier to other reasons.
I see the A5 has the following: "Versions: A500F (Global), A500FU (Europe), A500M (Peru, Colombia), A500Y (Australia, New Zealand), A500YZ (Taiwan), A500F1/A500K/A500S (Korea), A500FQ (Turkey)"
That's some 9 or more versions out. I did work in the cellular design center (engineering) so my bet is the models are region specific.
Maybe you are just learning that there is no global cellular standard. Each country has their own rules and carrier networks are all over the map.
Why do some phones only take sim cards from one region? I wanted to buy a Samsung A5 for use in Europe with a European card and the US with a US card, but this doesn't seem to be possible.

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