Many activists have been tracked via their mobile phones, and some countries conduct surveillance more extensively than others. You need to assess the risk for your own activities given the practices used in your country, how high-profile your work is, and what others in your community have experienced.
Phone companies have the capability to track and collect information about your use of mobile phones, including your location, and may share that information with the government if so requested. There is also the possibility of installing surveillance software on a phone that runs in the background without the user noticing. There is a risk of this, if your handset has been physically out of your hands for a period of time.