We strongly recommend you procure a mobile device management tool to manage your devices, the data used on the devices, the apps used, and the time spent on each app. Dimagi has an MDM solution you can consider called Focus that is tailor-made for international development projects with low-or-no connectivity. We have seen large i-NGO's use MDM solutions to secure their patient/user data, save money with restricted data usage and increase their frontline productivity.
CommCare works on two mobile systems- Android smartphones and J2ME/Java/Nokia-style feature phones. This article provides links to our best practices for procuring and managing such devices for both small- and large-scale projects.
Main page: CommCare for Feature Phones
SkillTrain in India-based social enterprise that develops training resources for vocational trainings like mobile phone or other electronics repair. They have videos available on their YouTube channel (in Hindi) that address some basic skills like transferring files between mobile phone and computer, and basic use of smartphones, in addition to advanced mobile repair techniques.