JMP will support 911 emergency calls for new users if a sufficient number of JMP users (at least several dozen) commit to adding $2/month to their JMP payments for 911 support (those who don't want 911 support won't see a pricing change). By default 911 is not activated for JMP users, primarily because the majority of users uses cell phones that obviously don't have a fixed location and thus storing some street address in JMP's database would be counter-productive.
It is recommended to use the phone's built-in emergency calling which goes through the cell carrier and enables the carrier to fill-in the location information based on cell triangulation. This does not require an active cell plan - only a cell phone - as all carriers (at least in North America) are legally required to let you place emergency calls on their network whether you're a customer or not (and your phone will automatically connect to their network to place an emergency call).