logging of incoming phone calls

I have an overpayment case where my client maintains that she made disclosure of a material fact by phone to ESA in July 2016.  The DWP claim that there is no record of this call (and have produced a list of incoming and outgoing calls which they say did take place).  They imply that their record keeping (via their CAM system - customer account management) is complete and infallible.

Is there any evidence I can access which throws doubt upon this?  e.g. evidence from a whistleblower, or an internal memo pulling staff up for not properly recording incoming calls.