Actually, I am sure that ANI is what is being spoofed here. (I have received calls from the same group myself.) ANI can be spoofed if the originating carrier allows, which is common practice for high-volume outbound automated calling campaigns. It is usually used legitimately to provide a number via which the called party can call back later if they miss the call or are disconnected.

(I work for a company which legitimately performs this sort of high-volume outbound calling.)

One other thing to note - this is actually the jursidiction of the FCC, not the FBI (at least not yet). As soon as you can prove that there is some sort of actual fraud going on beyond just violating FCC rules, then they might get involved.