Is your device using more data than normal, yet you haven’t really changed your levels of online activity? This could indicate that a spyware is installed on your device.
Lower-quality spy tools send as much data about your device back to their home base as they possibly can. This can use large amounts of data. However, some sneaky apps are much more selective with what info they look for and transmit back, making detection a bit tougher. It comes down to making sure that you stay aware of your monthly data usage and pay close attention to any sudden spikes.
Data usage can be easily checked on your device, without having to contact the telecom/internet service provider.