Some modems have the capability of monitoring the phone line to "see" how good the connection is If the line quality is poor, these modems can "retrain"--they change their equalization so as to better accommodate the lines See also equalization
If you retrain, or if someone retrains you, you learn new skills, especially in order to get a new job. Why not retrain for a job which will make you happier? Union leaders have called upon the government to help retrain workers. + retraining re·train·ing measures such as the retraining of the workforce at their place of work. to learn or to teach someone the skills that are needed to do a different job