The scammer is on the phone to you.
Their accomplice is on the phone to your bank, pretending to be you.
Your bank send you the notification.
You accept, and scammers proceed to drain your account.
Someone has just lost £18,000 because of this.
https://www.reddit.com/r/UKPersonalFinance/comments/1cih3kd/been_scammed_over_18000_through_my_chase_account/
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