There's another scam call doing the rounds. I had it yesterday.
In this one, the call is an automated voice telling you that you have been charged £300 from your Visa card within the last few minutes (I think they said 15 minutes) and then gives the receiver an option to press the number one button to confirm the purchase or to push two to speak with a member of staff.
Their con is to obviously get the mark to press 2 to speak with someone.
I didn't get that far into the process to find out what happens after the person is brought to the 'help desk', but I would imagine it's possibly a way to get their Visa card details.
That's just a guess. It could end up going down the computer scam rabbit hole where they remotely access the person's computer to visit the bank's website.
Whatever it is, there isn't £300 missing from a credit or debit card.