No No No, you did misunderstand.
You must install the spy app on the target device (your son's phone). Do not install the target app (other names: spy app, client app) on your own phone. This app silently tracks all activities of your son's phone and sent it to the server. So it must be installed on the target device - not your own device.
If you want to install one app on your own phone, it is a monitor app (other names: server app). This app is for loading and showing the data which recored and sent from the target device (the device of your son). You can export the data or do something with this app, but just for viewing instead of accessing the user control panel website.
So, the spy app works and depends on the device/phone. It does not depend on a phone number.
For more details, you can see another session about the target app, monitor app, the user control panel at Get Started -> All applications and websites in our system.