Have an old telephone with a line cord terminating in a 4-prong plug? This is the part you need to plug your phone into any modern RJ11 plug (the standard modular plug). If your phone's line cord terminates in 3 or 4 spade-tips on red, green, and yellow wires then you need another part and not the one here (click on this link to get to that adapter). This is becoming a very hard part to find, and when these are gone I suspect that is the end. Very few left: only one per customer please.