What is the call sign for a Army First Sergeant?

There is no standard radio call sign for any unit member- the call signs change frequently. The First Sgt is a Master Sargeant (E-8), with an (US Army) insignia of three stripes, three rockers, and a diamond shape in the middle. The position (senior NCO of a company, battery or troop) may be filled by an NCO of lesser rank, such as SFC or SSG.