Karen Bosold is a Madison mother with two daughters attending UW-Madison. She made sure that the girls took their cell phones with them to campus, and she reminded them to use the phones to call 911 if they get in trouble.

"That's the reason you get your daughter a cell phone," Bosold said. "They all tell you it's to talk to their friends. As a parent paying the bills, safety is the reason."

Now, however, Bosold is wondering if her daughters really are safer with their cell phones.

Bosold is not alone. Many parents are asking that question in the wake of new revelations about the killing of UW-Madison student Brittany Zimmermann — news that a call to 911 was made from her cell phone around the time of her death.

So far, the facts authorities have disclosed about that call are few. The official explanation from the Dane County 911 Center is that the dispatcher, who has not been named, answered Zimmermann's call, heard nothing on the other end of the line, and hung up. The dispatcher became busy with other calls and did not call back to Zimmermann's phone, according to 911 officials.

Because it is the policy of the 911 center to call back on such "hang-up" calls, Joseph Norwick, the center's director, said the handling of the call is being investigated.

But there are other troubling questions. Many of them go beyond the 911 center's handling of the call and have to do with the limits of cell phone technology. A visit to the 911 center on Friday and a demonstration of what happens when a cell call is received there shows there are limits to the information received from such a call, especially if there is no voice contact with the caller.

Even Madison Police Chief Noble Wray appears to have overestimated the capabilities of the system when it comes to finding a caller who has dialed 911 on a cell phone. Thursday, Wray said the 911 center had sufficient technology to track cell phone locations.

Friday, however, Tom Hanrahan, support services supervisor for the 911 center, said that even if a cell phone had global positioning system technology — the capability to use satellites to determine location — a 911 dispatcher can narrow down the position of a cell phone caller to only about a city block in urban areas. In rural areas, the capabilities are even less precise, possibly several miles.

Other information displayed from cell phone calls includes the phone number, the name of the cell phone company, and the address of the cell phone tower through which the call is being routed, rather than the precise address of the call.

Such inadequacies exist, Hanrahan said, even though Dane County is using the best technology available. The county upgraded to "enhanced 911'" in 2006 so that some identifying information is transmitted in the signals of emergency calls from cell phones. Today, he added, about 60 percent of 911 calls come from cell phones.

It's important, he said, for cell phone callers to be aware of the system's limitations.

"We waited and waited for this (enhanced 911) to come along in 2006," Hanrahan said. "It was a godsend. But it has its shortcomings . . . As far as locating accuracy, this is the best there is right now.''

Hanrahan said getting a pinpoint location from a cell phone caller requires a conversation between the caller and the dispatcher. A call to 911 from a land line, on the other hand, automatically displays the street address at which the telephone is being used.

Dispatchers with the 911 center are well aware of the system's limitations when it comes to cell phones.

Paul Logan, a county dispatcher since 1993 and president of the Wisconsin Chapter of the Association of Public Safety Communications Officials, said it's wrong to suggest that dispatchers always know where cell phone calls originate.

"We don't know where everybody is," Logan said. "Don't bet your life on technology."

Joe Farren, vice president of public affairs for CTIA-The Wireless Association, the nation's largest trade association for cell phone providers, agreed that cell phone technology and 911 calls are imperfect.

"These are signals that travel through the air," Farren said. "It's not precise. It's important for everyone using cell phones to know."

Even so, Hanrahan and other public safety communications officials, said the use of cell phones has greatly improved public safety by putting people in closer touch with help when they need it.

"They have saved many, many lives," Farren said.

Joel DeSpain, spokesman for the Madison Police Department, said the department recommends the use of cell phones to increase personal safety.

"We would advise the parents of college students to have their children use cell phones with GPS as a safety tool and to call 911 if they need to. We believe that's good practice on the streets," DeSpain said.

The issue is especially compelling on campus, both because of the tragedy of Zimmermann's murder and because so many students rely on cell phones. A 2007 UW-Madison survey of 1,246 students found that 85 percent use some form of cell phone.

Students at the university's Memorial Union on Friday said their confidence in the emergency 911 system is shaken.

"That's the one place where when you have an emergency, you need to call," said Stephen Delaney, a sophomore. "To know your lifeline isn't there, it's frightening."

Delaney said he hoped the incident was a fluke and said he would continue to call 911 in emergencies. But some students say they are beginning to question whether it would be wise to call a friend or a family member instead.

"I would rather call one of my friends rather than 911 because they're actually going to call you back," said Alexandra Leland, a freshman. "I never felt that way before."

In conversations across campus, students are wondering if Zimmermann's death could have been prevented if the response from the 911 center had played out differently.

"The whole thing makes me nervous," said Maggie Seidenstricker, a junior who lives off campus. "Just the thought that the whole thing could have been prevented. It's the 'What if?' What if the dispatcher had followed through? I'm not trying to point fingers or place blame. Whenever something tragic happens, you always wonder what if this or what if that?''

Dean of Students Lori Berquam said Zimmermann's ill-fated 911 call was news to the university Thursday, but it hasn't prompted any action.

"Brittany did everything right," she said. "There's no message we could give students that Brittany herself didn't employ."

Berquam said she has been in touch with Zimmermann's boyfriend and family, who are "wrestling with the new information and trying to make sense of it."

"Quite honestly, it brings up the question of what system do you trust?" she said. "This is supposed to be a system that's supposed to work."

Bosold, the mother who wrote a letter to the State Journal, said she hopes the police department will better enforce the policy of following up with all discontinued or dead calls. The dispatch center received 115 hang-up calls, including 83 from cell phones, on the day of Zimmermann's death.

"If it saves one person's life, what's 115 crank calls?" Bosold asked.

State Journal reporter Chris Rickert contributed to this article.