The RJ11 interface is very similar to the RJ45 interface, with the RJ11 being 4 or 6 pins (RJ45 is 8 pins). In computer systems, RJ11 is mainly used to connect to modem modems. For example, a 6-pole socket can connect RJ11 (1 pair), RJ14 (2 pairs) or RJ25C (3 pairs); an 8-pole socket can connect RJ61C (4 pairs) and RJ48C.
eBay