Actually its not the cell phone, but the 911 office. Not all 911 offices have the technology to trace cell phone, but all do carry tech to trace home base line. You need to find out if the city you live if they trace cell phones or not. Our agency, just to cross their t and dot thier i, make regulation that all homes must have home base line, better safe then sorry.
Here is an article written in Nov of 2007 in Missouri.
Suburban Journals | News | St. Charles County: 911 dispatchers can't trace cell phone calls
It only mention that state, but many states have lots of counties or cities that do not have the tech, to trace cell phone. My friend lives in a small town and cell phones can not be traced in her area. Plus there is also less towers in small city or towns, which make it harder to trace cell phone.