As of today, there is no efficient way of doing consistent monitoring with the phone number or an IMEI number.
If you want to monitor an Android-based phone, you will need to install mSpy directly on that device. That means physical access is required.
If you want to monitor an iPhone/iPad, you need the child's iCloud username and password to synchronise the cloud storage with the software. To successfully set up mSpy, make sure you have physical access to the device during the installation, and the iCloud backup is enabled on that device.
If you need more information, please refer to the following page: https://www.mspy.com/compatibility.html