CLeo

I have gaps in my NICs for 2015-2017. In this period I was non-employed.I would like to pay the voluntary contribution for class 3. I have calculated how much I need to pay. I have tried to speak to HMRC to request the 18 digit reference number but they are very busy. Can I write the reference number as NI+IC and my surname? Which account should I pay by bank transfer to? Thank you.