As they are for most forms of Internet fraud, the latest phone schemes are also difficult to track and investigate because of their frequency, their layers of anonymity and their global nature. Several investigators could not name a successful prosecution of the latest wave of phone swindles, though cybercriminals who committed other forms of fraud have been arrested.

In Tarrant County, Tex., the phone lines of several emergency dispatch centers were disabled in the last year because of a surge of automated calls, said Wanda S. McCarley, director of operations for the Tarrant County 911 District, which includes Fort Worth. The attacks lasted up to an hour and were aimed at 10-digit phone emergency numbers at the centers, which are accessible to callers outside the area, rather than 911 lines, which are not.

Something similar happened to a Texas hospital two years ago, when an intensive care unit’s phone lines were disabled for about six hours, said the chief information officer for the hospital chain that owns the facility, who spoke on the condition that he not be named to avoid unwanted attention for his employer. To defend itself, the hospital started using a service from SecureLogix, a telephone-security company, which the hospital’s chief information officer said had been effective.

In both cases, employees at the facilities were contacted by callers who said they were debt collectors seeking repayment of loans taken out by the workers. If the employees did not pay up, the callers threatened, the lines at the employees’ workplaces would be brought down. The attackers then overwhelmed the lines with repeated calls, causing busy signals for legitimate callers.

It is not clear how or why the specific employees were chosen, though law enforcement officials believe that swindlers in such cases may find names on public staff directories or professional sites like LinkedIn.