Recent advances in smartphone technology have elevated their potential as digital assistive technologies (AT) for blind or partially sighted (BPS) and deaf or hard of hearing (DHH) individuals. However, there is a gap in fully understanding the use of smartphones as AT and their impact on the quality of life (QoL) of BPS and DHH individuals. To address this gap, we conducted a mixed-methods longitudinal study over six months with 193 participants in Kenya. The study involved a baseline survey, smartphone digital skills training, and a follow-up survey and interviews to examine the impact of smartphones as AT. The findings emphasise the significant impact of smartphones on their quality of life, including impact on their identity and well-being, social inclusion and leisure, access to information and education, and material well-being. Building on the findings, we contribute an AT Impact Framework, which highlights the behaviours enabled by smartphones and their impact on the individual and their wider ecosystem. We discuss the applications of the AT Impact Framework to assess the impact on QoL outcomes of AT interventions and offer recommendations for policymakers, researchers, and designers.