The 302-900 telephone handset splitter is a device that allows you to connect two telephone handsets to a single telephone. The best way to use a 302-900 telephone handset splitter depends on your specific needs and preferences.

Here are some steps to follow to use the splitter:

1. Connect the splitter to the handset phone jack
2. Plug the two telephone handsets into the two splitter ports
3. Make a call and test the connection: Each handset should be able to hear and speak to the third party on the other end.

Overall, the 302-900 telephone handset splitter is a cost-effective solution for using multiple handsets with a single phone. It is a simple and cost effective method of training sales agents and customer service agents.  Ideal for telephone marketing.

Just be sure to follow the instructions carefully and test the connection to ensure the best possible performance.