Questions: How do I register the target phone if the app is already installed and hidden?
Answer: You should show the app (show icon) by making a call with the phone number ‘access code’.
An ‘access code’ normally is the secret number you defined in the user control panel ( https://my.iSpyoo.com), click the menu Device Settings Sync Settings, and you can see the access code. Also, you can change it for more security.
Default value is #1234* or #4779*
This is a link you can see for more detail: https://help.ispyoo.com/hc/en-us/community/topics