I haven't been hijacked or the car hasn't been ******

I haven't been hijacked or the car hasn't been ******. But sometimes I switch on the tracker when I go to the car and switch on ignition I don't get a call. And there's a time when I switch off tracker on my app and return to a car to switch on ignition but Car tracker agent calls. I asked the agent why is it happening he responded and said it could be my cellphone network