I paid back 10 years of class 2 volantary contributions. Now I pay 6,000 baht a year. I also have 7 years from when I was at school and university, and unemployed.
I only worked in the UK for 4 years but will still get a full pension, its a great return on investment. I only found out a few years ago that I could pay back 10 years for 65,000 baht.