Wait several days after the deadline the employer has set, and then follow up with a nice, polite phone call. Do not act angry that the employer has not kept his or her deadline for the phone call. Employers have many other jobs to do and while you being employed is of paramount importance to you, it is only another job to an employer. Exhibiting anger will assure that you will lose the job. Don't be surprised if the employer does not call you about the job.
Employers are people.
They are busy, certainly, but they also don't all know how to give disappointing news. Consequently, often an employer will just not follow through rather than tell you that you did not get the job. This does not mean that there is something wrong with you, so don't take it to heart. Evaluate your performance and see if you can improve it for the next interview. Remember, looking for a job is a competition.