Tech, when it comes to making the new trainee make the call post-solution (not sure what he said, but I guess it was generally, "I made an error, I take responsibility, I'm sorry" etc), I can see where there would be a variety of perspectives at your place. If your workmate thought it was a good idea, well, maybe that chap or gal thought it was good for the newbie to experience the gravity of the situation and that could be chalked up as valuable work experience and learning from mistakes, and that is all wonderful stuff. But it just came across to me and a couple of others that the boss who enforced the call by the newbie AFTER THE PROB WAS FIXED seemed like a public punishment of an individual--and individual who also happens to be at your company, with an integrity to own. Not sure which culture you are speaking from, but the integrity I work within involves recognizing that a person (maybe a newbie) is identifiable as the person who should have been able to do better, but you know what, we are going to shield that person from a ton of bricks falling on him, let's allow him to learn from this. That error-doing newbie will feel bad enough, he needs support to rise up from that bad experience.
So really there could be a counter-argument to what I and others said, as this could be seen as an opportunity for a new person to TAKE OWNERSHIP. But it all depends on the culture of the people we are talking about and the culture of the workplace. From my perspective, it is not good teamwork to allow all the bricks to fall on one person. That is what I think about employee relations.
When it comes to the relationship between solutions provider and customer, I still think that you don't go back and revisit a sitcheeation that has already been fixed. If you call someone a bad name one week and make amends, you don't say the next week, by the way I called you a bad name last week and I should not have done that. The suspicion on the receiving end will be that you still think of that person as the bad name. Your customer felt repaired by you one week when a prob was resolved. A boss figure had the newbie responsible for a prob call up and talk about the prob. The emphasis is on the prob (your company has a "newbie" and he needs to take responsibility for his wrongdoing) instead of "wow, we took care of you, that problem is in the past, and if you thought we fell short at one point, that is in the past, let's be ready for you in the future"