Any person who, from January 1, 1960, and thereafter, was a member of the call system of the North Providence fire department, is entitled to credit as service for the various purposes of the retirement system for one year for every three (3) years served as a member of the call system; provided, that the person pays into the retirement system in the manner, at the times and in an amount that the retirement board may prescribe, (1) for purchases requested on or before June 30, 2012, an amount based upon his or her compensation for the last year of each three (3) year period at the time of purchase of the credit and regular interest as defined in chapter 8 of Title 36; and (2) for purchases requested on or after July 1, 2012, the full actuarial cost of the time being purchased.