Owners of the Google Home smart speaker use a simple phrase to call on Google Assistant to handle tasks. The virtual personal assistant can control smart appliances inside the house, confirm an airline flight, get sports scores, news or the weather, all by saying a simple "Hey Google." That is different that the "Ok Google" phrase used by Android smartphone owners to activate Google Assistant.
Soon, though, the "Hey Google" command will call up the virtual assistant on handsets. Google is in the midst of sending out a notification to Android users telling them that Google Assistant can be trained to respond to both "Hey Google" and "Ok Google." It is a matter of syllables. The latter has 4 and the former has 3. That makes it easier and quicker to say "Hey Google." And while we are talking about mere fractions of a second, it should be interesting to see how many Android users decide to train Google Assistant on their phone to open with the "Hey Google" phrase.
So keep on the lookout for the notification telling you that the "Hey Google" hotword is now available on your phone. You can try to manually get the notification by opening the Google Play Store, and clicking on the hamburger menu. Click on My apps & games, and scroll down to the Google app. Look to see if there is a prompt for an update. If so, click on it and you should soon get the notification that "Hey Google" is ready to be used as a vocal command to open Google Assistant on your handset.