IPhone activation lock. How to unlock?

Use Apple's iForgot service, or contact their Account Security team, or if you’re the iPhone’s original owner, take it and its purchase receipt to a physical Apple Store.

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Why don't you do what I've suggested? Go to https://iforgot.apple.com to retrieve the ID and reset the password.

Assuming the Apple ID on your phone isn't the same as the one you log in here with (which you know or you wouldn't be posting), go, as already indicated, to https://iforgot.apple.com to retrieve the ID and reset the password.

Then the next step is to go to https://support.apple.com/en-us/HT204169 , select your country and proceed to contact Support: explain that you have an Apple ID you don't know.

Otherwise, and provided that you bought the phone new and not used, take it, the original purchase documents, and some identification to an actual Apple Store.

If you bought the phone used, then only the person who set the lock can unlock it.

Are you talking about the link I gave for Support? I just checked it and it works OK for me. If you can't get it to work (clear your browser's cache) you will have, if possible, to take the phone to an Apple Store as I suggested above.

I'm not sure what you mean, but I can't help you except to suggest you go to an Apple Store.

Niel's answer above gives you all the information you need to proceed.