Should a voluntary contribution show on my NI record

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I guess my question is..... given that I was contracted out for a number of years, what are the possible reasons that my NI record on Gov Gateway would show as complete for the purposes of SP for each year of my working life ? 

Because whether or not  you were contracted out, they are full (qualifying) years.....

If you were contracted out, what you were contracted out of was SERPS/S2P - you were still paying NI at a rate sufficient to qualify you for Basic State Pension (pre 6/4/16).

Contracting out for DB Schemes ended  from 6/4/16 and for DC  Schemes  from 6 April 2012.

If your "starting amount" for NSP (calculated 6/4/16) was less than a full NSP and you had a number of years before SPA, the relevant number of contributions or credits for those succeeding years could make them "qualifying years" to bring you to an amount equal to (but not in excess of) a full NSP.

 Even if you have reached the full NSP but are still working and under SPA, you will still be required to pay NI if earning the relevant amount.

https://www.gov.uk/national-insurance