It was Trump on the line, who told him he wanted new icebreakers.
While these conversations are often policy-focused, sometimes they are simply friendly chats over a shared passion - golf.
It's Trump's favourite pastime and a sport for which Stubb once represented Finland as a member of its national team.
"The golf really matters," said Brent Nelsen, a former professor of Stubb's at Furman University in Georgia, at which the Finnish president studied on a golf scholarship in the late 1980s and early 1990s. "He's a really, really good golfer."
The importance of Stubb and Trump's shared interest was highlighted in March when Stubb made a surprise appearance at Trump's Mar-a-Lago estate in Florida to play with the US president, South Carolina Republican Senator Lindsey Graham, and famed South African golfer Gary Player.
Instantly, Trump was wowed by the Finn's abilities.
On Truth Social, Trump wrote that he is "a very good player" and that the two won a member-guest tournament together.
"Trump loved that. Alex [Stubb] said he had a really good round," said Mr Nelson, who has remained in close contact with his former student and now friend, with whom he penned a textbook about the EU.
"Trump is always impressed with superior talent. Somebody who has some gift that really makes him stand out."
Over 18 holes on the sunny golf course, the two men chatted over a wide range of subjects, ranging from icebreakers to Ukraine. This time together, Stubb later recalled, allowed the two men to get a feel for one another.
"You learn about the personality of a human being on the first hole of a golf course," he told Bloomberg. "You see if they're edgy, if they're impatient, if they're fun, if they're relaxed. I have to say, I had no uncomfortable moments during that round."
Over lunch, Trump asked the Finn whether he could trust Putin, a man who Trump has repeatedly claimed to have a "great relationship" with.
"You cannot," Stubb replied, he told Bloomberg.
Within hours, Trump told NBC that he was "pissed off" and "very angry" with Putin - his harshest and most pointed criticism of the Russian leader at that point of Trump's second term.
Mr Nelsen said that the conversation obviously convinced the president.
"Alex spent seven hours with him in that golf cart," he said. "That had some impact on what Trump was thinking."
Trump's most recent Oval Office meeting with Stubb came just one day after Trump announced a ceasefire agreement between Hamas and Israel.
"Nobody in history has solved eight wars in a period of nine months, and I've stopped eight wars," Trump said in the meeting.
"I did it because I saved a lot of lives, and that's the thing that bothers me so much about Russia-Ukraine."
Stubb sat silently as he spoke, and just a few minutes later, reporters were ushered from the room.
But behind closed doors and away from the cameras, Stubb said he and Trump once again discussed the war and the Finn conveyed messages from Zelensky, although he did not provide any details.
A more detailed chat on Ukraine, it seems, will have to wait.