This Two-line RJ11 Telephone Cord Adapter (Line Splitter, T Jack, Y Divider, T-Adaptor L1 L2) divides one 2-line phone jack of your telephone base into two 1-line phone jacks. In other words it combines two wall jacks into one 2-line jack.  The adapter will convert two standard 1 line jacks into Line 1 and Line 2. (If you have 4 phone cables going to a 4-line base with two jacks then you will need two 2-line splitters.)