Don't scoff. This worked. Over 200,000 people were called. Polk County reported an unusually high return rate and, as I recall, received some type of special acknowledgement from the Census Bureau. I don't recall hearing much about citizen complaints (although there are almost always complains with any large notification, regardless of its nature).
Here are the tricks: The county had their census notifications set on a low priority so that any emergency notifications would take over the system and pre-empt the census notifications. And, they took their time about issuing the notifications. They weren't done in a fast burst, so no real threat of overloading the telephone infrastructure.
It's always a touchy topic when a system intended for emergencies is used for non-emergency purposes. In this case, county officials discussed it and decided that the reward outweighed the risk...the primary risk being that citizens would complain, or begin to ignore phone calls from the county. Neither turned out to be a problem.
Now, if they had only used the system to coach people how to fill out their vote ballots.
All the best,