The answer is: costumer service agent.
Although, arguably, no human profession, skill, or job, is a simple matter of execution, some are fitter to be learnt while being executed. One of these is costumer service. A customer service agent is in charge of assisting and providing advice for those people who have purchased a company´s product or services. Since the agent needs to deal personally with each different client, there is no singular guideline to be learned abstractly; rather, the agent needs to learn how to treat costumers in general in order to reach singular compromises for each client, therefore, there is no better way to learn this skill than to practice in by executing it, that is to say, it only requires on-the-job training.