What happens if I went from full time US employment to full time UK employment? Is that Class 3 or Class 2? I have contacted the International Pension Centre to find out what to do next. I'll also call US social security (eyeroll) to find out how to start now. I think I only have to worry about the issue raised by dunrovin once I start collecting my UK pension. I did download that document, and that language regarding the reduction appears to refer to WEP (again, which only applies once I start taking my UK pension, I think). In which case, my income will be much lower than it is while I'm working (so the penalty shouldn't be so high).
I sure do have a lot of annoying phone calls ahead of me, but it's best not to ignore it.
I recall reading elsewhere when this was brought up that in order to pay Class 2 NICs for years working in the US, you must have worked in the UK prior, for a specific amount of time - not a lot, I vaguely recall something like a couple of years. So, if you started your FT work career in the US, then moved to the UK, as far as I know, you can't pay Class 2 NICs for the years you were in the US.
This is just my recollection, so you can search for further information - this is not definitive information, and it doesn't address the issue of paying it retrospectively. I know I queried it when I first moved back to the UK, and they said I would have to pay Class 3. I'm pretty sure that was incorrect, but at the time I didn't have the time or money to pay it and then too much water pased under the bridge.
I've pasted one section from HMRC below, in case it's useful:
"Before you apply, you should check your eligibility to pay.
If you’re living abroad, working abroad, or living and working abroad, you may be able to pay voluntary Class 2 or voluntary Class 3 National Insurance contributions if you have either:
previously lived in the UK for 3 years in a row
paid at least 3 years of National Insurance contributions"
- so it may be dependent only on whether you had lived in the UK for 3 years. Best place to start is to Google "Social Security abroad: NI38" and read through this HMRC information.