If you call me to solicit something, you must be prepared to give me your real name, your address or telephone number, and if I've told you to stop and you call me again any time within the next 5 years you'll be having an encounter with police shortly.
It's not a solicitation under the law, because calls "to any person with whom the caller has an established business relationship" are expressly excepted by the law. I assume that's why they tried to use the CFAA instead.
And why not? They violated the CFAA: "Whoever ... knowingly causes the transmission of a program, information, code, or command, and as a result of such conduct, intentionally causes damage without authorization, to" "a computer ... which is used in interstate or foreign commerce or communication". The CoA ruled that the union knew that the volume of information would be harmful, and it reversed the district court's dismissal of the lawsuit. And they specifically targeted the sales department, which indicates that they were trying to interfere with the business's ability to function.
Because use of the phone system is authorized implicitly for phone calls, subject to restrictions under laws such as the one I mentioned. Using the description of "a program, information, code, or command" to describe a phone call with an audio message is a stretch, and was not intended to be covered by that law. It only technically falls under it because practically anything can be classified as "information". The definition of "damage" is also ridiculously broad, to the point where it would plainly cover even minor inconvenience. Charge them under the law that was intended to cover this sort of behavior, don't make insanely broad interpretations of a law that was intended for this. That will not set a good precedent. Seems like we don't even need robocall laws anymore if this law is going to be interpreted in such an insanely broad way and applied to situations it was not intended to cover. The law of unintended consequences will certainly apply here.