Around 1960, Ladislav Sutnar created a booklet for AT&T’s Bell System called How to Show Telephone Numbers On Letterheads, introducing the format for long-distance numbers we know today: (212) 222-2222. This booklet contains 15 example letterheads with Sutnaresque designs with a few different options for displaying 10-digit telephone numbers. From the introduction:

This booklet contains some new contemporary letterhead designs for business and personal stationery. / A variety of ways to display telephone numbers consistent with attractive design is shown. / For maximum usefulness and clarity always show all 10 numerals of the number [include the Area Code]. / The Area Code may be identified by using the words “Area Code” and separated from the local number by extra space, as shown in this book. Or, if space is limited, the Area Code may be identified by setting it in parentheses. / It may be appropriate to use the word “telephone” to distinguish the telephone number from other numbers on the letterhead. / Place the letters “TWX” before the ten-digit teletypewriter number.

Check out my scan of the booklet.