Millions of voters have reportedly received automated calls telling them to "stay safe and stay home".
Americans are voting in one of the most divisive presidential polls in decades, pitting incumbent Republican Donald Trump against Democrat Joe Biden.
The origins of the calls remain unclear, and some have not specifically mentioned voting.
"There's a little bit of confusion about this one across the industry," Giulia Porter, vice president at RoboKiller, a company that fights robocalls, told the Reuters news agency.
One of the calls reportedly says: "Hello. This is just a test call. Time to stay home. Stay safe and stay home."
This call that been doing the rounds for almost a year, but became one of the biggest spam calls in the country on Tuesday, Ms Porter said.
Officials have raised concerns over robocalls in the key battleground state Michigan, including one urging residents in the city of Flint to "vote tomorrow" because of long queues.
"Obviously this is FALSE and an effort to suppress the vote," Michigan Attorney General Dana Nessel tweeted. "Don't fall for it."
Janaka Stucky, a Democratic voter in Massachusetts, said he had received a robocall early on election day.
"My first thought was that actually it was a municipal test call for a Covid lockdown thing," he told Reuters.
"The more I thought about it I was like, oh this actually feels really off and weird and then started to feel like it was some sort of, maybe, voter suppression effort," he added.
New York State officials are also investigating allegations of robocalls spreading disinformation and encouraging people to stay home.
"Attempts to hinder voters from exercising their right to cast their ballots are disheartening, disturbing, and wrong," New York Attorney General Letitia James said.
The FBI has said it is aware of reports of robocalls but has not commented further.