Need HELP to Unlock my Samsung Galaxy A15. I am abroad

[Charm]

Hello, can someone/anyone assist me please? I had to purchase a new phone while visiting Edinburgh, Scotland last summer as my phone just mysteriously died when I arrived there. It was traumatic! Nevertheless, I am back in the USA and would like to use the phone I purchased there with different carrier here in the US. That carrier, AT&T, gave me a SIM but it is not working on the phone, and now they are telling me I have to unlock the phone to get the SIM to work. I tried to follow steps found online but none worked either. Any ideas on how I could fix this will be greatly appreciated. Thanks in advance.

[MI5 · Accepted solution]

@Charm 

If you purchased the phone from O2 it is not locked.

Guide: Unlocking an O2 phone to use a different SIM card 

If purchased elsewhere, contact the retailer / network for help.

[Charm]

Hello, thanks for your response. I purchased the phone directly from an O2 store there.

[Bambino]

@Charm Your phone may not be compatible with the US networks. You might want to speak to Samsung:

Contact Us | Official Samsung Support US

[MI5]

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@Charm wrote:

Hello, thanks for your response. I purchased the phone directly from an O2 store there.

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@Charm 

In that case, the phone is not locked and will work fine on any GSM network anywhere (which includes AT&T.

There is something wrong if the sim isn't working but it will not be due to a locked device.