Confused about paying NI shortfall. How to get 18 digit reference number

Hello. I'm a UK citizen recently returned to live permanently in UK. I reached state pension age in 2018. I have a new online account with HMRC. I was able to view my National Insurance record and it shows I have 8 years of NI contributions prior to moving to USA in 1979. According to my NI record I am eligible to pay for up to 12 missing years to qualify for State pension. Last year I contacted HMRC and was told that I could pay for the missing years and they would issue an 18 digit reference number. I put that temporarily on pause until I was sure how many years I could afford to pay. In the meantime I read online that I may be eligible to pay Class 2 contributions instead of the Class 3 amounts showing for each missing year on my record. I submitted a CF83 form and a cover letter as I was self employed in the US for 40 years. On my HMRC account it shows that the form was received 10/11/2023. No other information. I called again yesterday to HMRC helpline then referred to International pensions and also Future pension Center for information and update and was told that I had to request a letter confirming which years I could pay for and that letter would include amounts for each year, payment options and an 18 digit reference number, the person I spoke with submitted the request but said it would take 20 weeks to receive that letter. I'm ready to pay for the missing years but I'm concerned I've not navigated this process correctly/very well or that the information I've been given is accurate. Would appreciate any advise about how best to proceed. Thank you.

Hi Brian,
As you are over UK State Pension Age, I can confirm that the International Pension Centre would be able to check your National Insurance account and let you know the amount of voluntary contributions that you may pay to improve your entitlement to a UK State Pension.  From the details that you have given, it would seem that you have spoken with International Pension Centre so they will be in touch with you.  Just to confirm their telephone number is 0191 218 7777.
Thank you.
 

Please can someone tell me how to get the 18 digit national insurance payment number. There are no links on the HMRC app, nor HMRC website and the chat bot has not helped. I have also phoned HMRC and their chat bot keeps sending me the wrong links and when I hold on for a human the call then disconnects. I have my NI number and my state pension forecast figures, and all I now want to do is pay the voluntary contributions but am unable to, I believe, without the 18 digit ref. I live in the UK. Please help!! I have waited on phones for hours on end and trawled numerous web pages :

Hi,
You must first speak to DWP (Future Pension Centre) before making any voluntary National insurance payments to HMRC, if you are unable to get through on their helpline then you can request an interest in paying by completing the call back request form.
If you submit a request by the deadline, you will be able to pay voluntary NICs after the deadline has passed at the rates that were applicable in the 2023 to 2024 tax year.
To be able to make a voluntary class 3 payments from a UK bank account you must use an 18 digit reference number. We are experiencing a high volume of calls at the current time but we are taking measures to reduce the wait times. The quietest time to call is the first and last hour the phone lines are open so between 8-9 am and 5-6 pm.  
If you are unable to get through on the helpline to get your 18 digit reference number then you can contact us via webchat, please find the link below:
Ask HMRC online
Alternatively, you could pay via cheque and if you don’t have an 18 digit reference you can write you National Insurance number on the back of the cheque. 
You must make the cheque payable to 'HM Revenue and Customs only' and along with the cheque send in a cover letter with the following information:

• your name
• your address
• your phone number
• your National Insurance Number
• what the payment is for - Class 3 National Insurance
• the period you are paying for
• a request for a receipt - if you want confirmation of the payment

The address you will need to send the cheque into is:
National Insurance Contributions & Employer Office
HM Revenue & Customs
BX5 5BD
Thank you
 

I have live overseas and received a letter from HMRC regarding my voluntary NI payments. I went online to pay and it asked for an 18 digit reference number: a) No chance of getting through to the number in the UK - impossible and very expensive for me overseas - I gave up after nearly an hour 2. Ask HMRC online - no help - diverts to an advisor 3. Advisor service does not work. I have spent literally hours today trying to seek out the 18 digit reference number. How do we get it please - running out of options. Thanks.

Hi Charlotte,
I am sorry that you are finding it difficult to get through to us, though I would keep trying.
You can use the web chat function through HMRC to obtain a 18 digit reference number. 
You can contact us for payment details on 0300 200 3500.
You can also write in to request payment details. The address is:
PT Operations North East England
HM Revenue & Customs
BX9 1AN
Here is the link to use web chat: Contact HMRC
You could if not already have one, set up a Personal tax account: sign in or set up
Thank you.