A signal doesn't need to be excessively loud to be annoying. Pink noise, or even a 1 khz square wave, at whatever the maximum level that will get passed through the system would be an unpleasant listening experience.
But the real problem with this whole thread is that your retaliation is focused on the wrong person. The man or women calling is just an employee, trying to pay their bills by earning a paycheck. And the work that they happen to have been able to get is at a call center, where the job is to call whoever their boss tells them to. I doubt that anyone aspired to do that kind of work. It's the kind of job that people take while going to school, or until something better comes along. The things that have been mentioned in this thread will just make them hate their job even more than they already do. That may make them quit sooner, but the call center will quickly put someone else in their place. The people we really need to target are the call center managers and owners, and their customers.
And their customers are why they are calling in the first place. So, anything that can reduce their customer base will reduce the profitability of the call center, hopefully enough to put them out of business.
What I do is to get the name of their customer as quickly as possible. Then ask them to please tell their customer that, since I find their method of advertising distasteful, that I will be sure to take my business to their competition.