not very, if at all. While many states have laws in place require VIOP to be able to locate, it is not as effective as a 911 call from land lines. Within the past year, I was a 911 telecommnicator. The VIOP company wanted us to sign a contract indicating that they could just tell us a general location (as opposed to a specific location). Our 911 director refused to sign. Always be prepared to give a physical location.