A standard 8 pin telephone connector. ISDN lines are generally delivered by the phone company on RJ45's. On a standard U-interface ISDN installations, signal will be on the middle two pins (4 & 5) and phase is not important.
A standard 8-pin conductor modular plug. The RJ45 connector is replacing the RJ11 (6-pin) connector for use in 10base-T networks. RJ45 connectors look very similar to the old RJ11 modular jack used on telephones.