Avast bricked my phone

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I bought my phone used 2 years ago. Never signed up for Avast. Yesterday my phone announced that it was stolen or lost, repeatedly, and I had to take the battery out to make it stop. Every time I put the battery in it tells me that Avast has frozen my phone and I need to enter a PIN.
I've never signed up for Avast, so I don't have a PIN.
What do I do?

[Eddy]

It doesn't matter if you signed up or not.
You (or someone with access to the phone), installed AT on it.
The pin is for AT and as I said, it doesn't matter if you signed up or not.

https://www.avast.com/faq.php?article=AVKB214

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So how do I unbrick my phone?
And what triggered it to think it was stolen?

[Eddy]

See the FAQ I posted on what to do.

It can be caused by (not limited to)
- a SIM change
- bad contact between the SIM and the phone
- someone initiating the lost command

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I don't see a question or answer that applies to my issue.
Please re-read my original issue.

[Eddy]

No, please read the FAQ.
There is > "Avast Anti-Theft for Android: Controlling lost or stolen mobile device remotely"
and > "Can I restore a forgotten Avast PIN?"

There is also > "Avast Anti-Theft for Android: Restoring PIN"
https://www.avast.com/faq.php?article=AVKB127

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i'm with Mark E Williams my wife also gets the message also Mark it seems the official avast lock screen shows (locked by Avast)
and she also did not sign up for avast... i'm wondering has this problem anything to do with avast at all?

[Eddy]

It can only have to do with avast if someone installed avast on the phone.
If it is not installed, it can't be avast
However saying that, avast does have agreements with others to allow them to use (parts of) avast so it can be that it looks like the message is from avast while in fact it it from a third party.

First thing to do is checking if avast mobile and anti-theft are installed I would say.