I do not think they would contact past employers. Takes too much time and is not useful to them. There is not a place to enter a phone number, unless you put it in the job description box. It only asks for name of company, position, hours, start and end date, and a job description.
What kind of job was it? Was it just a retail job or something important?
Some schools state they want a dentist letter, but will take letters from mentor, or places you volunteered at, bosses, etc.
Others only say they want a dentist letter and do not say if they will take a letter from someone else to substitute it. Not sure how strict they are on this.