Wireless services use radio transmissions and coverage is not available everywhere. Quality of service may be affected by conditions within and beyond our control, including atmospheric, geographic, or topographic conditions, or by damage to your mobile phone or device. We do not warrant or guarantee that service will be available at any specific time or geographic location, or that service will be provided without interruption. Any statements or maps provided by us, our agents or dealers about coverage are intended to provide high-level estimates of our coverage areas when using our service outdoors under optimal conditions and do not mean that service will be available under all circumstances, at all times or without interruption. Estimating wireless coverage and signal strength is not an exact science. There are gaps in coverage within our estimated coverage areas that, along with other factors both within and beyond our control, may result in service interruptions, slower data speeds, or lower quality of service.
You should therefore never solely rely on your mobile phone for emergency calls, including to 911.
FROM: Virgin Mobile USA website here's the link