二共同相互通話

06:16 Sep 3, 2015 

Interestingly the 'party' (shared) telephone line' still survives to date in the following forms:
a) single line to a home with multiple terminals in various rooms to take any incoming call from nearly anywhere;
b) In call centers, multiple terminals ringing/blinking in response to the same incoming call, to speed up response time from the first available customer service officer.
In Case a) the incoming call is not electronically protected from eavesdropping by anyone in the home;
In Case b) since the call is being handled by staff sharing the same interests, privacy is not a priority; in fact the call is usually overtly monitored/recorded to assess and improve customer service officers' performance but, once answered, it is of course automatically isolated from all others to proceed confidentially. HTH

06:07 Sep 3, 2015 

Champak16, what era does your text refer to? 1930s or current?
二共同相互通話 appears to date back to the early days of telephony, when that communication network was hardly developed and 1 telephone line had to be shared by a local group of subscribers, particularly in the countryside.
Each farm may have had a terminal but, by default any subscriber could hear the telephone conversation in progress, if one felt like it. Apparently a 2+ step procedure was still available to establish a one-on-one private connection and conversation within that system. Here is how:
"When talking over the telephone. the privacy of your conversation ceased if you accepted a call from one of your 'party line' subscriber that had dialled 112.
To converse [solely] with a subscriber with whom you were sharing a party line, you needed to dial a special phone number. To achieve this, you had to dial 112 first, on your own terminal, then hang up.
Next, even though your terminal rang soon after, you were not to pick up your receiver but stand by instead." (Unfortunately, Wikipedia cropped off the rest of the procedure outline.)