All ERP projects i.e. the adoption, implementation and assimilation of ERP systems done with the aim of getting the intended benefits from the ERP system. Moreover, many researches has shown that effective ERP assimilation plays greater role in order to achieve intended benefits of the ERP system. Since, ERP projects are very expensive, for the countries like Ethiopia; it is intolerable to accept failure to get return over investment (ROI) from such kind of projects. Thus, in order to reduce failures especially due to assimilation problems, and realize the intended benefit of the implemented ERP system, first it needs to be assimilated well with its user.This explanatory case based study assesses the ERP assimilation in Ethio telecom based on the technology acceptance model and theory of interference. Hence, it identify success factors. Perceived usefulness (PU), user attitude (UA), Knowledge transfer are found to be driving factors of ERP assimilation in Ethio telecom. In addition, learning and training, empowering users, separate operation support, promoting ERP system are external factors. Finally, this study proposes ERP assimilation model called TAMI.