His under-19 debut was as a replacement against France in Blois in February 2005, and he went on make his first start in the match against England at Myreside, Edinburgh, a fortnight later. He subsequently played in all five of Scotland's matches in the IRB’s 2005 Under 19 World Championship in Durban, South Africa, including another appearance against England. Calum, a first-team regular with Glasgow Hutchesons’ Aloysians, played in two of the Scotland under-21 team's warm-up matches in season 2005-2006 before going on to make his international debut as a replacement against England at Falkirk in February. He was subsequently in the Scotland squad for the IRB sevens tournaments in Paris and London in May and June, and he joined Scotland's under-21 world championship squad after the pool stage as a replacement for the injured John Beattie, playing as a substitute in the 46-14 win against Georgia in Issoire. Calum, a former pupil of Hutchesons' Grammar School, is a member of the Glasgow Rugby Academy and the West of Scotland Institute of Sport.