Charlotte Sherwin

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.