The Mobile Companion uses AT&T carrier service (4G, LTE) for connectivity similar to a phone. Certain areas will not have sufficient reception to get location updates and full functionality out of the Mobile Companion but voice & emergency calls should work as long as there's any reception.
The call quality can be affected by reception similar to a cell phone.
For location services the device both require sufficient AT&T service. GPS location services require a view of the sky and will not work indoors. Additionally WiFi based positioning is used in urban areas and when GPS is not available but this requires WiFi networks within range that can be used to pinpoint the device location.