Previous studies in the ambulatory care setting have shown inconsistent results in regard to, or with respect to pharmacist telephonic transitions of care (TOC) encounters and reduction in 30-day readmission rates. No studies that have been completed within an accountable care organization (ACO) evaluating the impact of telephonic TOC encounters performed by a pharmacist have been identified. The objective of this study was to analyze the impact of clinical pharmacy telephonic TOC encounters on readmission rates within a primary care-based ACO. In this retrospective chart review, data for those who had a pharmacist telephonic TOC encounter and those who had an attempt were collected. The primary outcome of this study was all-cause 30-day readmission rate. Secondary outcomes included 30-day readmission rate for targeted disease states, time to readmission, and readmission reason the same as previous discharge reason. For subjects who received a telephonic TOC encounter, pharmacist intervention type and provider acceptance of intervention(s) were described. For the final analysis, 154 encounters were included, 83 encounters in the telephonic TOC encounter group, and 71 did not receive a telephonic TOC encounter. The 30-day readmission rates were similar among those who received a telephonic TOC encounter and those who did not: the difference was not significant (15.7% vs. 28.2%; P = 0.059). There was also no statistical difference in the secondary outcomes. Even so, the results of this study suggest that performing a pharmacist telephonic TOC encounter in a primary care-based ACO setting has the potential to reduce 30-day readmission rates and further research appears to be warranted in this important area of practice.