It can happen to even the most careful cellphone user: the phantom call, placed accidentally while your phone is bouncing around in your pocket or purse.

But the so-called "butt dial" may have another unintended consequence -- monopolizing police resources.

Authorities in Evanston, Ill., suspect up to 20 percent of all their 911 calls come from butt dials, as many as 500 a month in the city, CBSChicago.com reports.

“I don’t think the public realizes how often this happens,” Evanston 911 Coordinator Perry Polinski told the TV station. "When you consider the number of these types of calls that we receive, it really takes our resources."

Emergency personnel can't just hang up when they realize they've been "butt dialed." They have to wait on the line until the call cuts off, then call the number back to make sure there wasn't a real emergency. And in some cases, police even need to be dispatched to the cellphone's location, if the user couldn't be contacted by phone.

One of the reasons mistaken calls to 911 are so common is that cell providers make it easy, through speed dial settings. And a call to 911 can go through even if the phone is locked.

The problem has cropped up across the country, CBSChicago.com reports, and officials have a simple message for cellphone users: Be more careful with your phone.