There's a way to track your device's history. One is to use the IMEI number. You can look up how to find this number here: https://priorityunlock.com/how-to-find-iphone-imei/. You might find out that the phone was blocked or blacklisted. There are also ways to deal with that. For example, if your phone is from AT&T (like mine) and it was unpaid for a long time, you can read how to unlock an unpaid ATT phone. Spoiler: the most obvious method is to actually contact AT&T and talk to them about the issue. Probably, they would be able to help you even if your problem is not exactly the same as I've described.
Tracking apps for phones are very important. For example, you have a teenager of 13 and someone online is trying to humiliate or worse, seduce him! You can monitor his social networks and take measures to protect your child in time. Another example is elderly parents. Many old people have health problems, paranoia, anxiety or post-traumatic stress disorder. How can you help them? You can track their cell phones and track their GPS location. So I'm with both hands in favor, for the existence of such apps.
