Unlocking very old phone. Is it still possible?

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I found an old LG GS101 feature phone (made in 2011) in a pile of junk. Surprisingly, it's still working fine, but it's locked to O2 (checked with another network SIM).

I put an O2 PAYG SIM in it, it found the O2 network, but I can't follow the unlocking instructions on this page: https://www.o2.co.uk/help/account/joining-and-leaving-o2/unlocking-an-o2-mobile-to-use-on-a-differen... . All calls, including 4445, are redirected to the top-up service.

Is there anything I can do to unlock this phone, please?

[jonsie]

A guide here

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

You need to use the phone with an O2 sim and make a few calls and send a few texts

Then follow the steps in the guide

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The form mentioned on that page seems dead. This is what it gives me:

{
  "statusCode": 404,
  "statusMessage": "Page Not Found",
  "stack": []
}

[jonsie]

You need to call customer service

Or use the social media links below

Guide: How to find help & contact O2