@Linkat,
The Census Bureau does indeed make follow up calls. It's part of their quality control program. They may read some of your responses to you.
There have been items about this in the news recently.
It's not new and you are free to refuse to respond.
They don't call everybody, it's somewhat random.
I'm some ways, it's like the forms were in 1980 when a small percentage got a long form of the census which had weird questions (how many stairs are in your home/building....that is one I will never forget...or understand why it was asked)
Past personal experience: 1980, I received one phone call and it's main purpose was to verify the number of people living at the address.
2000 census was extensive. It also included a letter from the bureau which told me who would be conducting the phone interview.
Among other things, she asked about neighboring addresses where forms had not been returned from. In one of them, I knew the people had moved out at about the time of the census.
There were (2000) monthly follow up calls which had additional questions, and I was allowed to not answer any which I was uncomfortable with (income, education, religious beliefs) then it stopped and a letter was sent thanking me for the participation.
Again,nobody is required to do any responding on follow up calls