DWP say they can't trace my call....

Hi all, I'm hoping someone on here will have dealt with a similar situation. Basically in Jan this year I called the tax credits helpline and was advised that I wasn't entitled to claim WTC. Fast forward to May of this year and a friend advises me that I've been given the wrong information. Lo and behold, when I ring the helpline again, I'm told that yes I AM entitled to WTC, which I am now receiving. HOWEVER, as the person on the intial call never went into my file, he never made a note of my call. I have raised a complaint and provided them with the time, date, duration etc of the call, which is clearly shown on my phone bill but they are saying that as my phone number comes up 'withheld' when I call out, they cannot trace it. End of.

Obviously I'm seriously unhappy about this as it's quite a lot of money they owe me based on wrong advice given at their end.

What can I do????

Please help! :(