To enable ISDN-type services/ features to be started via analog lines (e.g. call waiting when a line is busy, three-way conference calls, etc.), you must send a signal to the network before dialling the service code and/or phone number.  

This function can also be called directly by entering the code (). 

You have an external connection. 

Call the system menu. 

Select and confirm the menu item. 

Select and confirm the menu item. 

Enter the service code and/or the phone number.