In order to register for Math 125, you must have a current placement score of 85-100% on the Prep for Calculus math placement test.
To check your current placement score, see http://math.arizona.edu/academics/placement/exams/scores
Transfer credit for Calculus I does not make you eligible for Math 125. You must take the Prep for Calculus exam and place into Math 125 with a score of 85% or higher.
Math 125 is an accelerated Calculus I course, meeting only 3 days a week in class rather than the 5 days a week for Math 122A/B. Because this class has less face-time with instructors in class, we require a higher placement level.