Most subscribers would expect to have an exclusive exchange line (solely for their own use) although under the Post Office agreements for Telephone Service nearly all lines were classed as pp (potential party) meaning that they had a liability to share if the need arose.
To overcome a shortage of lineplant (cables between the subscriber and the exchange) a method of sharing two telephone numbers on a single line was devised. It relied upon a third wire, the signalling earth, in addition to the normal exchange pair. Two exchange numbers would be provided, one for the X subscriber as normal; and one for the Y, but with the wiring reversed.
For outgoing calls:
The X subscriber operates a press switch to apply an earth to the B-line to which the line relay in the exchange responds.
The Y subscriber operates a press switch to apply an earth to the A-line to which the line relay in the exchange responds.
As each number has its own calling equipment, the metering is also separate, with the exception that if for any reason the wiring gets reversed, the call will charge to the wrong party.
For incoming calls:
Ringing current is applied to the B-line of the X subscriber or to the A-line of the Y subscriber.
The system was far from ideal, due to no privacy between the two parties, possible problems with call charging and the limitation of only one incoming or one outgoing call at a time.
Analogue Carrier WB900
In the early Seventies, the 1+1 Carrier System (also known as WB900 Analogue Pair Gain System or more simply WB900) was introduced to overcome nearly all of the problems of shared service lines. Thus the X subscriber had the physical line back to the exchange and was known as the Audio Sub, and the Y subscriber was known as the Carrier Sub.
The 'Audio' subscriber's line had a filter unit at the D.P. (Distribution Point) e.g on the pole, and a more complex unit fitted on the M.A.R. (Miscellaneous Apparatus Rack) in the exchange.
The 'carrier' subscriber had an electronic conversion unit fitted next to the B.T. (Block Terminal) in their premises and a corresponding unit fitted on the M.A.R. in the exchange.
In practice, both the carrier and filter units on the M.A.R. were mounted on a single board, with a mounting plate serving up to 10 carrier systems.
This effectively allowed two simultaneous calls to be made or received on a single line with complete privacy, with the Audio Sub working as normal (at frequencies up to 3.4kHz) and the Carrier Sub operating between 19 and 96kHz, so as not to interfere with the usual line conditions.
The carrier equipment at the subscribers' premises was powered by a nickel-cadmium battery within the unit and was charged via the exchange battery when the telephone line was not in use. Over a period of time this became a problem for heavy usage lines whereby the battery was often insufficiently charged to correctly operate the unit. One solution was to provide a mains powered 9 volt supply sited next to the carrier unit. However, unlike the original shared service lines, carrier subscribers did not benefit from reduced rental charges although they often experienced an inferior quality of service due to intermittent faults with the conversion units.
Over the years the 1+1 system encountered further difficulties as subscribers wanted to use modems, modern phones and touch-tone signalling, all of which interfered with the operation of the units. Updated versions were produced to counter these effects, the last version being WB900/9.
Digital Carrier - DACS
During the Eighties, a successor to the WB900 was developed. The DACS or Digital Access Carrier System used digital technology to share two customers on the same line. It no longer had an Audio or Carrier element and it reduced the need for battery packs and power units. It was still not fully compatible will every type of customers' terminal equipment and would not operate with some of the emerging digital exchange 'star services', but was a far cry from the bell tinkling days of the 'original' party lines.
To summarize, the Analogue party line schemes saved many potential subscribers from having to join a waiting list, while the Digital schemes additionally gave the flexibility to enable telephone service to be provided on demand.