Using Guadian, you can protect the access to the sms with your secret code; you can access to the sms only inserting the secret code when the application starts.
In general, you can protect the access to any desiderate application and to start it the phone will require your secret code.

You just need to insert the application “Installer” in the protected applications list. In this way, to remove Guardian will be required to insert the secret code.

This behavior is handled by the parameter “Settings > Protect Applications”.
To avoid that phone requires the secret code anytime you try to use the application, they become free after the first time you insert the Secret code.
They will be protected again after a given time of phone inactivity. Changing the parameter “Settings > Protect Applications”, you can modify the inactivity time, assigning the value that you prefer, and eventually set to “Immediately” so that the secret code will be required every time.

Use the phone which you previously configured as recipient of the sim changed notification and send this sms to the phone where Guardian is currently installed:
*LOSTCODE*
Please note: if the sim in the phone protected by Guardian is authorized, then your secret code will be shown directly on display of the protected phone, while, if the sim is not authorized, you’ll receive an sms containing your secret code.

If you want insert a number not present in the phonebook, you should create a temporary entry in the phonebook, use it in Guardian and then delete the entry. However Guardian will keep the selected number.