Only call sign I ever hear is when they are co working in an Task Force with locals that they do not have everyday ops with. Then the only call I hear is.. *US MARSHAL, or just MARSHAL and then the last name.* If they are not interopping, they just like DEA, and FBI (when your lucky enough to hear them in the clear) only use names. USSS except the bodyguard protective details are the same way ... Protective Details on the other hand love their funny little code names and call signs. I have heard CBP, ICE, and USPI use call signs often.