HMRC told me voluntary class 2 NI contributions "for last six years only" - why?

I phoned HMRC and DwP and I was told by both that I could only make voluntary class 2 NI contributions for "last six years" (or "last six complete tax years"). However, Social Security (Contributions) Regulation 61B states that I can go back as far as 2006-07: https://www.legislation.gov.uk/uksi/2001/1004/regulation/61B Please confirm that what was I told on the phone is wrong, I'd like to send copies of my self-employed invoices for 2006 to 2013 to HMRC. What is the correct address?

Hi Westholme,
Can you please contact National Insurance self employed department so they can investigate your account and advise accordingly against your circumstances.
As you have been informed you do not meet the conditions to pay class 2 for those specific tax years previously, you can write in with your query on regulation 61b or call the helpline and the advisor can send a technical referral.
Number - 0300 200 3500
Overseas - +44 191 203 7010
Open - Monday to Friday: 8am to 6pm, Closed on Saturdays, Sundays and Bank Holidays.
Address:
HM Revenue & Customs
National Insurance Contributions Office
BX9 1AN
Thank you.