We carry telephone handset Y adapter that allows two handsets or headsets using the smaller RJ 6 plugs to be connected to a regular telephone at the same time.  Great for real time training. 

 Line Isolator is used to prevent interruptions and disconnection's of your telephone conversations, credit card / debit terminals or Internet sessions. When telephone line is in use, the line isolator prevents all other telephones and equipment from having access until the call has ended.  Isolator required on each telephone or device you want to control interference from.

2