The prop made his first Scotland appearance aged only 21 in the 2018 Six Nations and made another two appearances against USA and Canada by the time he 22. McCallum couldn't build on that early success and he only made a handful of appearances for Edinburgh before his contract ran out at the end of last season.
Danny Wilson handed McCallum the lifeline of a short-term contract until the end of the year by Glasgow Warriors, and he's focused on making the absolute most of this chance to resurrect his career.
"We'll see what the boys upstairs think after a few games," he told The Evening Times. "I could be here for the year, I could be elsewhere, we'll just have to wait and see. But my focus at the moment is on these six months and however many opportunities I get."
With Zander Fagerson, Simon Berghan, Enrique Pieretto and academy prospect Murphy Walker Alison available for Glasgow, McCallum knows competition for game-time will be intense but he says he is okay with that because he believes that he will, at least, get a fair crack at the whip.
"It was the first time I'd been in a shit fight with other players and there were some times when I was frustrated and I maybe didn't know how to handle it," he says, reflecting on time in Edinburgh. "I thought I was doing everything I could, but who knows? Sometimes it's your word against the coach's and at the end of the day he picks the team. Sometimes I struggled to deal with that.
"It did get a bit twitchy at times, particularly in the Covid era when clubs are a bit more strapped for cash and are maybe trimming back squad sizes. It's a case of who's got space or who needs someone like me? And lucky enough it was Glasgow who came in for me, who are in the Champions Cup this year, and who have made some really exciting signings and have two Lions boys coming back."
"So, I've got an opportunity here after landing on my feet to assert myself on the big stage. With Zander and Enrique coming back soon, and Simon here as well, they are three top class internationals that I get to push with, under a new coach in a new environment.
"I need to focus on my game and not worry about anything else apart from what the coaches want me to do. Danny has been quite clear in what he wants me to do and that helps me massively. He knows what my strengths are, what I need to work on and what he wants me to bring to the squad.
"It's about having that love to learn again - and that's been reinvigorated by joining Glasgow.
"You can get into a rut where you're only playing once in a while and put so much pressure on yourself that you almost forget what you should be doing because you're so focused on trying to get picked for the next game. I just need to sack that and focus on the game I'm playing in.
"I'd hope time is on my side at 25. I certainly don't think I've cracked it. I might not have cracked it at 35!"