A "Community Dial Office" (CDO) was a small Class 5 telephone exchange in a rural area.
These most often provided capacity for 1,000 or fewer customers and were designed for unattended operation.
[2] Any operator assistance or toll call billing for these unattended stations was handled remotely at a larger exchange in the city.
In the 1960s, Northern Electric developed the SA-1 community dial office as a small Class 5 crossbar switch for local use in rural areas.
[3] In the late 1970s, Western Electric's number 3 Electronic Switching System (ESS) was installed in small communities with up to 4,500 telephone lines; it also served as a workhorse in some large private branch exchange installations.