Billionaire Elon Musk joined President-elect Trump’s call earlier this week with Ukrainian President Volodymyr Zelensky, a sign of how Musk’s close relationship with Trump could manifest itself as he prepares to take the White House.

Since the election, Mr. Musk has used his social media company to talk up how bright the future will be under the president-elect.

The billionaire owner of SpaceX was on the call briefly and his involvement wasn’t planned, according to those familiar with the conversation.