This Columbia Daily Tribune article from 1971 reports General Telephone's exhortation to residents in the Central Missouri city to dial all seven digits. Some fair use excerpts from the article:

A growing town means more telephone dialing for Columbians.

General Telephone Co. of the Midwest is running a campaign to get its local subscribers to dial all seven digits of local telephone numbers. R.P. Randall, division service manager, says there is but one goal: giving better service.

The General Telephone effort is both obvious and subtle--obvious with advertisements suggesting, "Dial all seven digits," and subtle when the operator asks for your complete telephone number.

[...]

Once you could dial three digits and reach another Columbia telephone. Then it took four. Most recently it has been five. But growth has now made it necessary to dial seven numbers.

"The smaller a community is, the smaller the number of digits people have to dial," Randall says.

All that really changed when General telephone opened two branch offices, one ... for numbers beginning with 445 and one ... for numbers beginning with 474.

[...]

"We're rapidly approaching the time when everybody will have to dial all seven digits," Randall says. "I would say that within the next five years we will have to dial all seven digits no matter where you're caling within the Columbia district."

Already there is a waste of time for customers who live in the 445 exchange and fail to dial all seven numbers of a telephone in the 474 district, for instance. By building the habit of dailing [sic] all seven digits, Randall says, "The customer is going to get better service."

[...]

"If people would dial all the digits," Randall says, "there would be no trouble to anybody."

Interestingly enough, though, five-digit dialing was still possible in Columbia's three original telephone exchanges (442, 443, 449) until November 1986 when new switching equipment was installed for all Columbia exchanges.