The Gers B team boss admitted he was frustrated with the first-half performance but was pleased with the reaction from the players after the break.
Speaking to RangersTV at full-time, he said: "In the first-half we were slightly frustrated just because we had a lot of possession but didn't really carry any threat or purpose with it.
"That was the conversation at half-time, whether it is 50,000 fans at Ibrox or whether it's 200 here you want to give some sort of entertainment but for all the possession we had first-half I don't think we threatened their backline often enough and I don't think we created enough opportunities.
"I thought the second-half was a clear change in that one which resulted in us creating much more opportunities and playing much more of the game in their half."
McCallum praised the impact of substitutes Robbie Ure and Tony Weston who were both involved in the second-half goals this afternoon.
He continued: "I think both of them brought energy to the front line, the second goal comes from Robbie putting a good bit of pressure on their centre-back and getting a regain and finding his way into the box.
"The third comes from Tony Weston lifting the ball in for Cole [McKinnon] to score. They always talk about when you come on as a substitute bring something to the game and I think they did that."