The Air Canada timetable for 25 Oct., 1970 lists five digit Zenith numbers for many localities in Ontario and Quebec.
In the United States, most Enterprise and Zenith numbers were four digits; some of each were five digit and some Florida places had a seven digit Enterprise number. A few places (N.J. and FL) used WX and four digits.
Boise, Idaho was listed as EN 722, possibly a typo, the only three digit number.
Mohawk Airlines (based in Utica, NY) had a similar mix of Enterprise and Zenith numbers in the April 1969 timetable. Regular numbers were a mixture of two letters + five digits, and seven digit numbers. Some exchanges could not be made into words (Buffalo was NF 2-3000).
In the October 1970 timetable, all regular numbers were given as
7-digits; (Buffalo 632-3000). A number of upstate locations now used a 1-800 number. I think someone suggested 1-800 numbers came into use about 1970.