In this era of communication technology, educational benefits and many ethical problems associated with using mobile technology devices have emerged in the education sector. This study explored the reasons for teachers' indulgence in the unprofessional use of mobile phones and its repercussions on education in Tanzania. The study was conducted using a qualitative approach drawing on a sample of 28 participants selected from the Dar es Salaam University College of Education. The data were collected through semi-structured interviews and focus group discussion methods. The findings revealed several reasons for teachers to use their mobile phones unprofessionally. They include the lack of policy on acceptable mobile phone use in schools, lack of professional training, lack of efficient induction and mentorship, and irregular in-service training on ethical matters. The study uncovered several repercussions of unprofessional use of mobile phones among teachers. These were the disruption of teaching and learning; increase in students' delinquent behaviour; hostility and quarrels in schools; and deterioration of the image and status of the teaching profession. The study recommends incorporating ethical issues related to ICT use in the teachers' code of conduct and integrating ethical literacy in using ICT devices in the teacher education curriculum. Moreover, concerted efforts should be taken to educate teachers on the ethical use of ICT devices to reduce the damage that threatens to undermine the government's attainment of the Education For All agenda and the development vision 2025.