Generally speaking, an Application programming interface is required for the integration of such services….
Amazon Alexa API (Alexa Voice Service)
> Amazon throws in $100 million, developer tools to open Echo’s Alexa ecosystem (Jun 2015)
> Cortana for all: Microsoft’s plan to put voice recognition behind anything (May 2015)
> Google Now API Opens Third-Party Support to Select Apps (Feb 2015)
> IBM Extends Reach of Watson API Portfolio (Feb 2015)
> Apple Tees Up Siri APIs in an Effort to Take On Google Now (Jun 2015)
See also my Quora question: