I've worked in call centers for many years and what you're describing is common practice among all inbound call centers (i.e. they take inbound calls as the majority of their business) because all time scheduled is actually forecast by interval (usually 30 min but some go as fine as 15 mins) based on historical data+any expected shifts in business (holidays, promotions, media coverage, etc.)
I don't pretend to be a lawyer but odds are there is nothing 'legally' you can do about it. Labor is 70% of any call centers' business, successfully unionizing will only prompt the company running it to shut it down or move the call center elsewhere. Keep in mind, many (most?) states do not have a law or regulation requiring a business give breaks at all so the fact that they are being allowed is often seen as 'more than enough' from a labor dispute standpoint. These tend to be the same states that do not support or protect unionized labor which is where many call centers will set up to avoid inflated wages.