Having worked for HMRC for 40 years, we never Rang anyone demanding money for any reason.
Any PAYE Tax underpayment had to be notified with a paper Tax Assessment for each year showing what was paid and what should have been paid and an explanation how it occurred.
PAYE underpayments were normally collected through the Tax Code the following year or years.
If the person wasn't paying any Tax in those future years, then after several years a decision was made to write the Tax off as uncollectable.
The HMRC website usually has details of current "Phishing" Scams they been made aware of.https://www.gov.uk/government/publicati ... s-examples