Google tests lots of things. Adding a call button to organic snippets makes sense on a mobile device. Less so on desktop.
What's more interesting to analyse is from where Google has pulled the number. Was it the meta description, opengraph, the page content, a widget. Did Google look for a 'tel' anchor? The schema would have been the best source but this business uses the useless Yoast schema generator and doesn't include the telephone number.
More research is needed.