Jue Li 50-port telephone distribution frame 6P4C2C crystal head 4-core 2-core telephone line voice 50 to 1U cabinet information distribution frame management frame RJ11 jumper audio management frame 50-door telephone line voice 50 to 1U cabinet information distribution frame management frame RJ11 jumper audio management frame 50-door telephone line voice 50 to 1U cabinet information distribution frame management frame RJ11 jumper Audio management frame 50-door telephone line