Portable interconnected Voice over Internet Protocol (VoIP) services can be used from virtually any Internet connection anywhere, which raises challenges for the emergency services community in determining the location from which a 911 call has originated. You should be aware that:  VoIP 911 calls may not connect to the PSAP, or may improperly ring to the administrative line of the PSAP [Public Safety Answering Point], which may not be staffed after hours, or by trained 911 operators.  VoIP 911 calls may correctly connect to the PSAP, but not automatically transmit the user's phone number and/or location information.  VoIP customers may need to provide location or other information to their VoIP providers, and update this information if they change locations, for their VoIP 911 service to function properly.  VoIP service may not work during a power outage, or when the Internet connection fails or becomes overloaded."
Federal Communications Commission: https://www.fcc.gov/