四线制话频电话中继器

voice frequency repeater office control telephone repeater station voice frequency telegraphy equipment voice frequency keying voice frequency relay voice frequency telegraph voice frequency ringing set voice frequency band four wire connector parallel four wire feeder high frequency telephone selective call with dual voice frequency voice-frequency repeater voice frequency repeater four wire repeater four-wire repeater telephone frequency repeater four-wire telephone circuit telephone repeater submerged telephone repeater two-wire telephone repeater telephone repeater station voice-frequency