SolvedCECCG15400 | England & Wales: Call in by the IOPC
From HM Revenue & Customs · Complaints from External Customers about the Conduct of HMRC staff Guidance
The IOPC can call-in for assessment particular cases of concern or sensitivity that might not otherwise be referred to the IOPC under the mandatory criteria. This may occur where the IOPC is notified directly by a complainant or other persons of a case which has not been referred or where media coverage has drawn attention to a case.
In practice, call-in by the IOPC is rare.