An RJ11 to RJ45 converter converts a telephone socket into a networking socket. An RJ11 is the socket used to plug in telephones, and an RJ45 socket and jack is for computer networking.
An RJ11 to RJ45 converter is necessary because the two registered jack types are different in size and structure. The RJ45 jack is larger because Ethernet networking cables contain more wires than telephone cables.
An RJ11 to RJ45 converter that is specifically used for voice over IP communication is called an analog telephone adapter. These devices allow a caller to use a telephone line for VoIP calling.