I was just reading about Trojan apps from the android app store called "grifthorse". There are numerous apps that are commonly downloaded and installed on android devices that seem perfectly innocent and useful. When installing and using the app, a picture of a present 🎁 pops up and says you've won a gift. It's an sms trojan and it ask for your phone number. Once you enter your number, it automatically subscribes you to a non existent service and charges your bill to your carrier. As these are third party charges, by entering your phone number gives implied consent and the journey of trying to undo them begins. They are not carrier charges, they are app charges that you have given unknown permission to be charged on your account. The play store has already removed hundreds of them, but many more are still installed on people's devices.
Read this article and if you happen to have any of the apps, you should Uninstall them immediately.