Different brands of phone trackers offer different services. Usually call logging, access to messages, e-mails, videos, pictures etc. Some tracking devices offer money back guarantee on trackers too! The purpose for which phone tracker is being used solely depends on the purpose of spying. Spying for illegal purposes can incur strict charges. FBI and police agencies claim that they obtain special permission from court for carrying out any such spying activities. The use of phone trackers can help a lot in cases of emergencies. It is easy to track a suspect by tracking its phone even if it lies under the rubble of earthquake.
When it comes to using a phone tracker for spying a loved one like a spouse who you think is cheating on you, your child who is getting aggressive or is skipping school for some absurd reason. You may feel guilty for invading their privacy no matter how close they are to you. But, you need peace in your life. Everybody deserves a fair share of truth, at least from people who you care about. If you have decided to get a tracker finally, then it can be assumed that the case is in its crucial stages. Finding out about what is worrying you will only help you build strong confidence and strong relationship if you are wrong luckily. At times one cannot endlessly wait for other to come clean and tell you the truth automatically. So in cases where it is important to restore a relationship or reveal a potential threat, the use of phone tracker is still considerable as long as the authority is not being misused. It totally depends on your interpretation whether the use of phone tracker is legal or not. As long as you prefer the use of phone tracker that it will not hurt the other person it is well and good.
One other serious implication of using phone tracker is moral violation. If the other person finds out about any such activities, it can be highly offending. It becomes a bigger issue if the person is sincere and you have been assuming things in the mean while. For such reasons, the use of phone tracker need a lot of secrecy and privacy.