As I covered at SEL yesterday, Google Posts has launched a call now button in Google Posts. It basically lets you add your phone number as a call to action in the Google Post for when your local knowledge panel shows up. So in the posts carousel, the call to action button can now dial your phone number.
Here is the option in Google My Business when creating a Google Post to select the call now button option:

It will show your business’s phone number from you Google My Business profile, I do not see a way to override that number without changing your business phone number in Google My Business.
Then, when it shows up in the Google search results, if you click on it, it will dial the number on mobile and when the call is done, Google asks for feedback on how that experience went.
Joy Hawkins who first spotted this shared the GIF of the experience for the searcher:

Google prompts you to leave a review after clicking the new call button on Posts.
— Joy Hawkins (@JoyanneHawkins) July 12, 2018
Forum discussion at Twitter.