This is absolutely spot on, with the caveat that you do need to disaggregate from accounts to people, which is the hard problem. Having people call a phone number is definitely not going to work as a way of achieving this disaggregation. I'm pretty sure I could create a system to bring that call center to a halt with fairly minimal cost in less than a week of coding.
As an attacker, you can also hire people in call centers to make phone calls at scale for you.
I think we may be talking about different things? I was just talking about a scaling problem of providing legal notifications of account suspensions and providing a means on getting them unsuspended. I wasn't talking about DoS attacks.
Lots of companies have call centers, so I'm not sure what you're envisioning here, or what financial gain there would be for spammers to DoS the call center. After all, their accounts are already getting suspended by the algorithms, regardless of whether innocent consumers have any appeal to this, and DoSing the call center won't help spammers get their accounts unsuspended.