RJ11, RJ25 all use six-position modular connectors. RJ11 is a physical interface, being used for single line POTS telephone jacks in most homes and offices. RJ25 is for a three line jack. An RJ11 jack is nearly always a 6P4C jack; two of the six positions to connect tip and ring, another two positions may carry low voltage AC or DC power.
RJ25 6P6C Vertical Type, IDC terminate