"OK Google" and "Google Now"

Trying to get Google Now to work consistently.  I've been able to configure "dialer" to be SIP extension on Asterisk and can back calls to other extensions.   "Ok Google" is intermittently recognized after a call - sometimes it works immediately and other times it never works. Zoiper does not work even after making Zoiper the default dialer.   When giving the command "call xyz" the option to call from GSM or Zoiper is presented.