drgeoff's answer applies to a WiFi connection that is stable and capable of supporting low-latency, low-jitter, low-dropout connections. A MiFi connected to a mobile phone network is likely to be unable to do that on any consistent basis. The mobile phone network uses voice CODECs that can tolerate some signal depredation. Think about the mobile phone calls you make over a weak connection, where the sound quality varies considerably, and may drop out frequently. The call sounds crappy, but you can still make do. OBiTALK devices use a CODEC (which was designed for stable, hardwired connections) that can't cope with those signal quality issues.
