Living and Working in Oz - Need to pay "Top Up" contributions & in one hit - How?

Hi All, I have 19 years paid NI and now have the opportunity to pay back to 2006 by April 2025. I wish to do this in one lump sum, not direct debit monthly, etc. Can you advise how best to do this? Also is there a way for UK Ex Pats to contact the HMRC without having to call the Telco? Is there a "chat" facility, email or preferably a messenger service like WhatsApp? Cheers

Hi Robin Boniwell,
If you’d like to pay voluntary National Insurance contributions whilst overseas, please read our leaflet NI38 ‘Social Security Abroad’ and then complete and return our CF83 application form.  
Both the NI38 and CF83 are available via:  Social Security abroad: NI38
If you’d like to contact us, our Helpline telephone number from overseas is +44 191 2037010 or our postal address is:  
HM Revenue & Customs,
PT Operations North East England,
BX9 1AN,
United Kingdom. 
Thank you.