Yes, apps to monitor phones are legal. However, there are some essential things to consider when using these services. First, it is important to consent from the person whose phone you are monitoring. This means that if you are a parent wanting to monitor your child's phone activity, you should make sure they know about the cell phone monitoring app and give their consent to allow you to use it. Additionally, depending on where you live and the laws in your region, there may be certain restrictions or regulations that need to be followed when using a phone monitoring service. It is important to understand these local laws before downloading any such app.