Prime is a standard callsign for the 317th Airlift Squadron (charleston). The numbers usually are picked by a scheduler / mission planner who schedule all the training flights for the week - numerology helps them remember what that flight was supposed to be doing … Usually has to do with the day of the week, the number of training flights that day, whether is formation, air refueling, DV on board, etc.
Bronze, Prime and Turtle – these are the new call signs now used by Reserve aircrews assigned to the 315th Airlift Wing at Joint Base Charleston, South Carolina.
“Our motto is ‘First in reserve,’” Davis added. “Our call sign comes from the word primus in Latin which means first.”
https://www.315aw.afrc.af.mil/News/Article-Display/Article/750177/charleston-reserve-squadrons-get-unique-call-signs/