This advertisement in the Mexico (Mo.) Ledger, December 17, 1971, alerted readers to upcoming changes to the phone service in nearby Centralia. Similar ads appeared in the two daily newspapers in Columbia, Missouri.
Centralia was in the trade areas of both Columbia and Mexico. While Centralia is in Boone County, just as Columbia is, it's actually a little bit closer to Mexico.
The same illustration of a disembodied hand and a telephone dial was used in all the ads that General Telephone placed to publicize the changeover.
Mexico was served by a different phone company, Southwestern Bell (now AT&T). Calls between Mexico and Centralia were long-distance calls. At the time, since they were in the same area code, such calls did not require inclusion of that portion of the phone number.