I checked today's (2/26/08) newspaper and found that even within a single telecommunications company advertisement there was no standardization or consistency. More below.
The old Bell System was very fussy about this sort of thing and liked consistency and standardization. Every user manual for equipment, both residential and commercial, contained a few pages on good telephone manners, and they published many booklets and films solely on telephone manners for individuals as well guides for businesses. In earlier years, using the telephone instead of a personal visit or a written letter was seen as cold and impersonal and the phone company sought to dispel that so as to increase telephone usage; however, they wanted the use of the telephone to be seen as respectable and proper, thus the frequent lessons in good manners. Also, naturally, they wanted to reduce wrong numbers and dialing errors and the use of Information and operator assistance for things the customer could do for themselves.
These days we all know how to use the phone technically (well, I don't have a clue as to how to dial overseas but I have no need to.) But I wish people, particularly in businesses, would be taught better telephone manners, including proper use of the HOLD key and giving progress reports to a caller waiting for someone to answer.
However back in the 1960s it was still trying to educate people about area codes and to use them, both in telling contacts one's home area code and to place calls by area code not by city name, so they really pushed the prominence of the area code. (In the 1950s people often didn't know a person or business's distant phone number, they'd call Long Distance and say "Get me John Smith Co. in Kansas City" and the long distance operator would get local information for the number first.
Verizon Wireless used three formats in the same ad:
1) In large type: "Call 1.800.555.2368"2) In tiny type--the phone number for each local store or agent in the area: "311-555-2368".
3) In medium type another toll free number: "800-555-2369"4) In the footnotes yet another toll free number: "1-888-555-2360"
So in one ad we have four different formats, including three different formats for toll free numbers.
(It is also curious that they have three different toll free numbers for their business.)
The Sprint wireless ad was all "311-555-2368" format except the general number "1-800-ABCDEFG" (letters used instead of numbers.)
The AT&T wireless ad only had phone numbers for local stores, no general toll free. Almost all stores were "(311) 555-2368", but a couple of stores were listed as "311-555-2368".