The content is only a part of it. We are a species specially-suited to pick up on others emotional states and factor that in our responses. A mismatch in response to certain emotions is going to cause further problems as, to an angry customer, it comes off as patronizing, dismissive, and disrespectful.
Typically, angry customers are conveying severity and urgency. If you do not respond with a similar sense of severity or urgency you now have two people who aren't on the same page. That's not a good foundation to solve problems on.
There's also good angry, where you can tell the person is angry, but not at the call center person.
I personally take care in this, saying things to reassure, such as indicating that I know it's the company's fault, not the rep's. This vecomes more of a "bartender" situation, where the anger is clearly at the situation, not the rep, and can create a more engaged, helpful response even.
I think one could sue for purposefully changing a person's appearance or voice, without their consent. And no, calling a help line can never ever have a tos enforced, as often the entity is required by law to deal with warranty or other issues. No tos applies.
Back to changing the voice, as others have mentioned, tenor, tone, etc all are part of speaking. They cannot be separated.
Tone is often, for example, brought up in testimony as it conveys something. It has as much meaning as the words.
I would be astonished if this isn't challenged soon.
As someone that's worked in call centers for a fair time in my life, I'm not sure about this.
I've worked for a cable company for a while that really turned to shit, and went into buying off representatives rather than fixing their technical problems (heavily investing in making city owned fiber illegal in many states). Customers being enraged with us was a very strong motivation for me to leave and find better work with a less abusive company. And yes, call centers commonly shut down/move/ or actually start giving better service when they can't keep staff because they are such a crap company that their users are in constant rage mode.
Typically, angry customers are conveying severity and urgency. If you do not respond with a similar sense of severity or urgency you now have two people who aren't on the same page. That's not a good foundation to solve problems on.