[1] However, it wouldn't be until the 2018–19 season that he would play for the Friars, as he spent parts of three years in the United States Hockey League (USHL) with the Youngstown Phantoms and Central Illinois Flying Aces/Bloomington Thunder.
The Friars would enter the tournament as the #4 ranked team in the nation, and would advance to the Frozen Four, where they'd lose to eventual champions Minnesota Duluth.
In addition to continuing to be a force on the Friars blue line, he took a major step forward in his offensive production, scoring five goals and 23 assists in 28 games.
[2] Unfortunately, Callahan was not able to replicate his on-ice success in the 2020–21 season, with three goals and eleven assists, but he was still a major part of a Friars team that made it to the semi-final of the 2021 Hockey East tournament.
On February 23, 2022, while still in the midst of his senior year of college, Callahan's rights were traded by the Coyotes to his hometown Boston Bruins in exchange for a seventh-round pick in the 2024 NHL Entry Draft.