The Federal Trade Commission says it is cracking down on robocalls, stepping up efforts to halt illegal automated sales calls that are a growing annoyance for millions of households.

Since September 2009, the FTC has prohibited prerecorded telemarketing calls unless consumers give written permission to receive them. Marketers are not allowed to trick people into signing a robocall waiver by burying it in fine print.

“American consumers have made it crystal clear that few things annoy them more than the billions of commercial telemarketing robocalls they receive every year,” FTC Chairman Jon Leibowitz said when the law was enacted.

Despite the ban, the calls are on the rise, the agency said.

Complaints about robocalls have risen steadily from a monthly total of just over 65,000 in October 2010 to 140,500 in September 2011, according to the FTC’s most recent figures.

The FTC attributes the spike to technology. Making automated calls is easy and increasingly cheap. The calls also are becoming more difficult for law enforcement and regulators to trace because of “spoofing” technology that allows the originator to transmit a fictitious caller ID.

The agency is planning an October summit in Washington, D.C., to explore ways to track and halt illegal robocalls.

Although automated sales calls are prohibited, other prerecorded calls are not, including those from politicians, survey takers, debt collectors and most charities.

Live sales calls are not illegal. To block such calls, list your phone numbers on the federal Do Not Call registry ( www.donotcall.gov or toll-free at 888-382-1222). Companies with an existing business relationship with a household are exempt from honoring the registry unless a customer requests it.