铃流 meaning in Chinese

ringdown
ringing current

Ringing and tone for telephone automatic switching network
电话自动交换网铃流和信号音
The test methods for the technical specifications of ringing and tone for automatic telephone switching network
电话自动交换网铃流和信号音技术指标测试方法
Technical requirements and test methods for ringing , power sources , alarm and distribuing frame in manual toll telephone switching equipments
人工长途电话交换设备铃流电源告警配线设备技术要求和测试方法