NI Gap in records of international student now British Citizen

hello I want to know what category (Class 2 or 3) is suitable for someone who came to UK as international student but is now a British citizen - please note the gap is only for the year the person started studying in UK, all others years are paid in full as the person started working part time 6 months into the uni course. The call center and support line for people who lived abroad wasn't much useful as advised to complete CF83 but the guidance for this form implies that this situation isn't applicable to the person many thanks

Hi,
The CF83 application is used to apply to pay voluntary National Insurance contributions (NICs) for periods spent abroad, therefore this would not be the correct form to complete for this situation.
Class 2 NICs can only be paid in the UK, when a person is self-employed.
Class 3 NICs would be suitable for a person who lives in the UK and was not employed or self-employed for the period they want to pay for.
Further information can be found here - Voluntary National Insurance: Overview
Before deciding to pay voluntary NICs, you must check with the Department for Work and Pensions, that paying voluntary NICs will increase your pension.
Their contact number is 0800 731 0175, lines are open from 8am to 6pm, Monday to Friday. 
Further information on how to pay, can be found here - Voluntary National Insurance: How and when to pay
Thank you.