Even though the smaller RJ12/RJ11 plugs fit, the outer edge is slightly thicker and can bend the next set of pins in the socket, so it is better to use an RJ45-RJ11 cable than use a RJ11 plug in an RJ45 socket.
RJ11 Modular Plug 6POS Important Notice : This data sheet and its contents (the “Information”) belong to the members of the Premier Farnell group of companies (the “Group”) or are licensed to it. No licence is granted for the use of it other than for information purposes in connection with the products to which it relates. The standard for telephone plugs varies from country to country, the RJ11 6p4c modular connector has become by far the most common used in the world. EXW telephone plug follow FCC standard. Plugs, cables, keystone jacks, inline couplers for a broad range of applications and markets with UL, ISO/IEC certification.