Experiences with 911 calls and SIP

Hi all, I'm having a discussion with one of my vendors about the 911 capabilities of their SIP VoIP phone system. The vendor says that if we use an Enhanced 911 service that their phone system will transmit location information to the PSAP at the time of the call in addition to the ANI. I was under the impression that this functionality was not possible, hence all the problems that Vonage is having. Can anyone help clarify this for me? Thanks, Dan