You have asked about paying voluntary Class 2 National Insurance contributions. Class 2 National Insurance contributions are paid by a person who is self employed. If you are self employed, you would pay these contributions through your Self Assessment. If you are self employed and need to register your self employment with HMRC you can use the link here:
Register for Self Assessment
If you are not self employed in the UK and you are enquiring about paying voluntary National Insurance contributions, it would be voluntary Class 3 National Insurance contributions that you would pay, not Class 2. Please see the following link on how to pay voluntary Class 3 National Insurance contributions, this includes information on how to pay by cheque.
Pay voluntary Class 3 National Insurance
If you are abroad you may be able to pay either Class 2 or Class 3 National Insurance contributions. See the following link for further information, and how to apply to pay voluntary National Insurance contributions whilst abroad:
Social Security abroad: NI38
Please note, before deciding to pay voluntary National Insurance contributions, you must contact the Department Of Work and Pensions to confirm that paying voluntary National Insurance contributions will increase your pension. If you do not, and your pension does not increase after paying them, you will not automatically be entitled to a refund. Their contact details arebelow:
0800 731 0175 for customers under state pension age.
0800 731 0469 for customers over state pension age.
+44 (0) 191 218 7777 for all customers living outside of the UK.