McCallum was named in the World Rugby Under-20 Championship Scotland U20 squad for 2015.
Despite progressing through the age-grade systems as a loosehead prop, he made the switch to tighthead prop and graduated from the BT Sport Scottish Rugby Academy, joining Edinburgh Rugby on a two-year contract.
He debuted for Edinburgh Rugby as a replacement in the 45-10 defeat of Treviso in October 2016, before making his first start for the club in the 59-17 victory over Timisoara Saracens the following week.
January 2018 saw McCallum being included as one of four uncapped players in Scotland's 40-man squad for the 2018 Six nations tournament.
The following year, he signed a two-year extension which will see him remain in Edinburgh until at least 2021.
As of June 2020, he has made 55 appearances and has scored 20 points for the Scottish club.
(LS June 29, 2020)
|2016 - present|
|2015 - 2017|
|2018 - 2019|