When this device is "on hook", the speech circuit is disabled & the ringing circuit is connected
[DOC] [TLE] Patent US2873316 - Party line lock-out circuit - Google Patente [PAR] It is, therefore, necessary to disable the signalling, or indicating, device on a party 
line ... diodes each have a breakdown voltage such that when all sets are on-
hook, ... the band of the speech currents, it is necessary to disable the ringing 
circuit of ...