A study of 105 university students examined whether the length or type of Auditory Progress Bar (APB) used in a telephone on-hold situation had an impact on users' subjective mental workload. Auditory cues can be designed into APBs to provide information to the caller regarding the approximate duration of the hold time. Previous research has found that the majority of callers tend to multi-task during on-hold situations. Thus the design of any stimuli presented while the caller is on hold should take this into account. To do this, it is important for designers to understand how APB design might impact callers' mental workloads. In the current study, participants experienced one of three APB design types (cello, sine, electronic) in four different lengths (30, 60, 120 and 240 seconds). An on-hold call center experience was simulated using a computer-generated interactive voice response system and workload data was collected using the NASA-TLX. The results of this study indicate that the type of APB does not appear to have a significant impact on callers' assessment of mental workload, but that the duration of the stimuli does.