In the USA the Government in the guise of the Federal Communications Commission has ordered VoIP providers who allow access through mobile devices to provide their customers with support for calling Emergency Services.
It is perfectly possible for providers to do this. What isn’t possible is the identification of caller location information. In the UK Ofcom has recognised this and specifically exempts users of mobile voip services from having to provide address details. The 999 system recognises a mobile VoIP number as such.
A mobile VoIP user doesn’t of course have to use a mobile handset for this to be the case. A laptop with a softphone is a more likely scenario with users travelling between different office locations.
VoIP providers in the USA are going to have a difficult time of it methinks. They don’t appear to have the same leeway as in the UK and the FCC isn’t telling them how to go about providing the location information.