Less than two weeks after Microsoft introduced its hands-free AI helper, Cortana, to Android mobile devices, the company has yanked the feature from the US market. Users used to be able to say "Hey, Cortana" and then issue a command, just like "OK, Google" natively does for Google Now. However, the feature appears to be incompatible with Google's voice recognition system and, in some cases, could make the phone unusable for commons tasks -- like making phone calls.