A study carried out in the largest telecommunication organization in the Netherlands focused on-the-job training (OJT) as an effective training method. It also studied which variables explained OJT effectiveness and whether improvement of OJT effectiveness was possible through modification of training design. The objective was to improve trainee telemarketers' sales communication with customers. A pretest/posttest design was used. The first posttest was immediately after training, the second approximately 15 weeks later. To study the effects of modification of training, two groups were formed: a treatment group of four regional call centers and a comparison group of three centers (total number of trainees was 60). For the treatment group, four training activities were added to the OJT: self-study assignments, observational checklists of other trainees' behavior, logs for trainees to formulate behavioral goals and evaluate their progress, and short meetings with managers to discuss progress in job behavior. Findings indicated a modest but significant progress in trainees' behavior following OJT. With regard to trainee characteristics, age, behavior, and perception of the training before the OJT predicted trainee progress. Two organizational factors that were predictors for progress were managerial support and workload during training. The length of OJT was a strong predictor for progress. A number of trainees in the treatment group did not participate in the additional activities. (Appendixes include 29 references and 2 tables.) (YLB)