Voluntary Class 2 payments changed to Class 3 without any correspondence

Hello. I have been paying voluntary Class 2 payments since 2009 (and since leaving the UK). On my 23/24 record the amount outstanding is 907.40 which equates to the Class 3 payment amount. Could this be an error? or would there be some reason for the change from Class 2 to Class 3 which I am not aware of? From my side nothing has changed. When I try and call from overseas the call always goes dead once it is answered, which is adding to my frustration. Any advice or help would be greatly appreciated. Thanks

Hi,
An individual’s Personal Tax Account will always show any shortfalls of voluntary National Insurance contributions at the class 3 rate.  
If you have received an annual request for payment for the 2023-2024 tax year then please use this to make any payments of voluntary contributions as this should be at the class 2 rate.
If you do not receive annual requests for payment then please complete and return our CF83 application form and you may either pay your voluntary contributions by direct debit (through a UK or Channel Islands bank account) or by annual payment.  
Please read leaflet NI38 ‘Social Security Abroad’ before completing the CF83 see:- Social Security abroad: NI38
Thank you.