"He just came straight out with it," he remembers. "There wasn't really much else to say. I think we were both in shock."
The reverberations are still being felt. Sam Chalmers had phoned his father to say he had failed a drugs test, taken when he was training with the Scotland under-20 side in Ireland. He had tested positive for the banned anabolic steroids methandienone and stanozolol. His case was investigated by an International Rugby Board judicial committee and, two weeks ago, the 19-year-old Melrose utility back was banned from the game for two years.
The teenager offered no defence, and nor does his father. "Sam was stupid and naive," Chalmers said. "He knew what he was doing and he got caught doing it. If you take these things, you know what might happen. Now he's got two years to think about what he's done and to figure out what to do next."
The offence brought the issue of drugs in Scottish rugby to the fore, with another unnamed player suggesting a typical club in Scotland would have at least two players regularly and knowingly taking illegal performance-enhancing products. The Scottish Rugby Union have stressed the extent of their testing programme - around 800 tests a year - but Chalmers casts doubt on its efficiency at amateur level.
"I coached Melrose for nine seasons and I never saw anyone taking anything," said the former Scotland and Lions fly-half who now coaches Surrey club Esher in England's National League 1. "However, it was pretty common knowledge that a lot of people were taking stuff at clubs around Scotland.
"It's easy to get hold of pills and supplements off the internet, and a lot of players have no idea what they might be taking. There is very little testing at that level of rugby. We could go a whole season at Melrose, one of the top sides in the country, without a single player being tested."
Chalmers emphasises he makes no excuses for his son. "Sam did a really silly thing and he is paying for it," he says. But he despairs of a rugby culture in which size has become the main criterion by which players are judged - a backdrop that, combined with a lax testing regime, is likely to see more and more young players yield to pharmacological temptation.
"I was having a conversation with a player from my generation recently," Chalmers says. "He had been among some current players, and all he heard about was weight and strength and power.
"They were all obsessed with these things and having little competitions for who could do the most squats or bench presses or whatever. There was no chat about who was the best kicker, or who could pass the ball best. All they were thinking about was making themselves bigger and stronger. Skills had nothing to do with it.
"One thing that never came out in Sam's case is that he has had a particular medical history that has made it very difficult to gain weight since he was a little boy. He's had multiple allergies all his life and he was diagnosed as a type 1 diabetic when he was 12.
"But rugby has been his entire focus since he was a kid. As well as the pressure he was putting on himself, he's also had to deal with pressure from other people just because of who he is. It's been tough for him; because his dad played for Scotland and the Lions there are a lot of expectations around him."
So how did he end up taking PRO-SD, the supplement that contained the banned agents and which, according to its own literature, should not be taken by anyone under the age of 21 or any athlete who is subject to drug testing?
"He always wanted to be bigger," Chalmers explains. "He was getting a lot of peer pressure. He was doing all right at the level of rugby he was at, but he missed out on a couple of selections to go higher. He felt he would have a much better chance if he could put on some weight.
"It was the end of the season for him. He told me he had a couple of friends who weren't involved in rugby anymore who had taken this stuff and had put on the sort of bulk he wanted. He knew it was wrong, but because he didn't think he would be playing for a few months he decided to give it a go.
"He took it for a couple of weeks, but because of his medical history he started to feel some ill-effects and stopped. Then he got called up to train with the under-20s, which is how he ended up being tested. And now he's got two years to think about it."
Chalmers is not sure how Sam will spend those two years. In his submission to the disciplinary tribunal, he offered to take part in player education to warn against drugs, but nothing has been confirmed yet. He might spend time travelling.
"It's going to be tough for him. I know he did wrong and I know people see these things in black and white, but it's not as if he stood to profit financially from what he did.
"Rugby has been everything to him and it's going to be hard for him to live without it. He'll feel that his right arm has been cut off. He's got to find other things to do now. His life has been built around training on Tuesdays and Thursdays and then playing at the weekend, so his entire routine will be different and it will not be easy to adapt."
Again, Chalmers says he is not arguing his son is innocent. Rather, he believes condemnation should be accompanied by some understanding of the factors that lead young players to drugs. It is the individual's responsibility to stay clean, but the sport has to change its culture.
"We've got to get away from this gym-bunny mentality," he says. "Coaches keep demanding bigger and bigger players, but their focus should be on skills. Look at the All Blacks: they're not all giants but they are the best team in the world. The emphasis has to change.
"Sam will serve his punishment, and hopefully he'll come back a better player and person. He did wrong, and he knows it. I know people will call him a drug cheat, but what I see is a kid who got frustrated about the game he loves and the way it is going."