Product Review
You are supposed to plug the headset into the connector and then the connector into the jack on the phone where the headset was plugged in, then you can plug in a 2nd headset in. You don't use it to split a phone line. The headset connector is a rj9 or rj10.
RJ48: 10P10C = 10 Position 10 Contact - used in some comms wiring and data environments
RJ45: 8P8C = 8 Position 8 Contact - used in Network Wiring including Cat 6 and Cat5e
RJ12: 6P6C = 6 Position 6 Conductor - used in System phones
RJ11: 6P4C = 6 Position 4 Conductor - used for ADSL, telephone, and modem cables etc.
RJ9 / RJ10 / RJ22: 4P4C = 4 Position 4 Conductor - used for Telephone Handset cables.