If a person in the Contacts application on iOS  cannot be found in one of Apple 's services, the application will try to use Google Hangouts  if the person has a Gmail  address.  This can be seen in the Call  and Video  buttons under the contact's name.   Version 16.0.0 of the application saw the ability to use the system's phone application to send and receive calls.  This integration is most certainly the source for this ability.        This contact only has a phone number and it's not associated with   any Apple services.  But it's known to Hangouts, so the video call   button is labeled "Hangouts."  
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