I would want to empower all employees to stop assisting a customer in anyway if they become verbally abusive. No talking, no helping, just straight pivot and walk away to get a manager to throw them out. If you are on a phone, you hang up the call right away and have the manager call them back on a recorded line and ban them from what ever service you provide.
If you are a manager, you must make it your duty to protect your workers and yourself over terrible attitudes from rude customers. No appeasing, no bending over backwards, no trying to message their tender egos. Just blunt and direct that their business is no longer appreciated.