Reverse boycott. Together we can destroy their cost/benefit ratio by consuming their service. Their legal model is less vulnerable than their business model. Individual efforts against their legal model have failed for years. United efforts against the robo-calling business model succeeds.
When a company can repeatedly call with the intent of getting money their customers can repeatedly answer or call back with the intent of not giving them any.
Don't ignore/block/report them. It doesn't work. These folks want you to ignore them for as long as you can stand to or until you give them something valuable like money or information. Ignoring them is being their good customer. Sending a cease and desist is giving information. It lets them know you remain their good customer. Preparing to initiate unlikely individual legal battles is being their good customer.
Be their bad customer. Make them talk to you fruitlessly for as long as they can stand to or until they stop selecting you as their customer. These companies cannot spend seconds much less minutes on the phone with every person who will never send them a dime. But they don't know who that is. You do. That knowledge is power. Every second you can keep their staff on the phone will render their business less profitable giving them a reason to never call you again.
I like to ask general questions they should but usually won't answer, "May I have the name and address of your agent for service of process?" Calmly and slowly ask them to spell every word in the address. Read it back for verification. Control the pace. If they are rushing then politely ask them to slowly repeat. "Are you a corporation and if so in which state are you incorporated?" Repeat your questions when you don't get direct answers. When they won't answer a question ask, "Would you like to comply with the business and professions codes of your state?" That is usually the point when they hang up on me but if they say they want to comply then begin your questions again.
Repeat while you have the spare time. These folks have many victims and few operators. If everyone answers or calls back but pays nothing the robo-calling business model quickly becomes unprofitable. Don't aide and comfort the enemy by ignoring them. Answer! Call! Have a nice long slow friendly chat! Make them hang up first.
For any business, customers who consume the most resources but don't pay/spend (or otherwise add value) become the least desirable. Most businesses will stop (cease and desist) soliciting an undesirable customer. Those that don't stop remain knowingly vulnerable to the actions of their undesirable customers. Few companies can afford to let everyone take a piece of them whenever they want endlessly. Call and consume their operator time without paying or giving information. Be a bad customer. The worst customer. Inspire them to invoke their right to refuse you service by not soliciting you.
Unite or remain conquered. Answer/return every call - become well practiced at keeping these folks on the phone - or count yourself not amongst the free.