JRT26

Hello HMRC I was employed for 38 years but my record shows that I have 4 years short as my employer contacted out. I contacted yourselves and have received my 16 digit reference to make payments for my gaps. Since leaving employment I have been self employed for 5 years and have not made any voluntary class 2 voluntary payments via my self assessment. I have subsequently been told by the future pension forecast helpline that I could make Class 2 voluntary payments instead of class 3 NI back payments to fill these gaps. If this is correct, please can you let me know how I can back date these class 2 contributions and for how many years. Will the 16 digit reference still be relevant for class 2 NI or was it particular to my class 3 NI enquiry. Thanks