Jellytots

Future pension told my husband he has 3years to pay to enable full state pension but there were two amounts he could be asked to pay but to contact hmrc who said they would write to him but this was months ago. My husband is thinking of just paying the 3 years at the highest amount so he's covered but doesn't have an 18 digit code how would he pay it . We live in the UK. Reading other people's comments would it be his NI number ic then his surname then the first letter from his first name if he needs to make up to 18