“Whenever people come up to me at home, there’s two things they tell me,” said Representative Billy Long, Republican of Missouri. “They say, ‘Support the president.’ The second thing they say is, ‘When are you going to do something about these robocalls?’”

Americans received 48 billion robocalls last year, and unsolicited dials are expected to make up almost half of all calls that Americans receive by the end of 2019, according to YouMail, a service that collects and analyzes calls through its robocall blocking service. Only a fifth of those calls are considered legitimate automated alerts, like a reminder of a coming appointment from a doctor’s office.

Attempts to defraud Americans through automated calls have grown more prevalent and more sophisticated. Robocallers can generate telephone numbers that mask their location, adopting area codes that mimic the neighborhood of the recipient, making it more likely people will answer. Some scammers have gone so far as to mimic hospital telephone numbers as a guise to ferret out sensitive patient information.

Nearly 7,000 external calls falsely identifying the callers as employees flooded the Moffitt Cancer Center in Tampa, Fla., in just 90 days, Dave Summitt, the center’s chief information security officer, testified. During that same period, the center received over 150 calls from phone numbers that falsely identified the callers as employees of federal agencies. In both cases, posing either as an employee of the center or a federal worker, the callers tried to obtain personal information about the patients and physicians.

“I am rather astonished that others can use our own phone number range, fraudulently represent our organization, and we have no recourse,” Mr. Summitt said.