Tax credit overpayment

I got a letter last week saying I have been overpaid nearly £5k tax credits which of course is not an amount I have to hand. It tells me to ring the number on the letter to sort out paying it etc but I also want to find out why I owe this money and if it is correct However 3 days of trying to ring the number on the form has resulted in 3 60 minute holds that were cut off. 2 50 minute+ holds that were cut off and 1 where I got through to an operator after 54 minutes who asked me my national insurance number and then....you guessed it I got cut off. How am I supposed to confirm anything let alone arrange to pay when your phone service does this? I have now been on hold total about 5.5 hours and have only heard a human voice for less than a minute. I need to speak to someone about this but it is impossible. Please help. This is very stressful having red letters sent to me when I cannot contact anyone.

Hi Andrew Cole,
Unfortunatly I am unable to provide an individual response as I am unable to review your records.
I suggest that you ring the Tax Credit helpline (not the payment helpline) on 0345 300 3900 and ask for an explanation of how the overpayment occurred
or write to the Tax Credit Office to ask for an explanation.
Tax Credit Office
HM Revenue and Customs
BX9 1LR
United Kingdom
Thank you.