Adam Kerman wrote:00
The "INTERCHAGEABLE" office codes were introduced in Chicago sometime around 1983 or so. "INTERCHANGEABLE" area odes came about over 10 years later in 1995.
Remember that ths history of Chicago (as well as certain parts of California and New York City) is Panel and #1XB, which did NOT need a1+ for 7D or 10D toll calls. SXS offices (with the exception of southern California's "SAMA" step offices) NEEDED an access prefix to route to the CAMA/Toll office. It was 112+ in most Bell (and many independent) areas starting in the mid/later 1950s where DDD was being impelemented, but by 1960, the recommendation was for 1+ which began to replace 112+.
But when N0X and N1X office codes began to be introduced circa 1983 or so, to differentiate between N0X/N1X-xxxx seven-digit calls within 312 vs. N0X/N1X+NXX-xxxx ten-digit calls to other area codes, rather than using a post dial delay timeout (which one could cancel with the # pound button at the end), Illinois Bell chose to use 1+ for ten-digit calls, while seven-digit calls had no prefix. And because the 1+ prefix differentiated any ambiguities, there were office codes assigned within312 (and later 708, etc) which conflicted with adjacent and even HOME area codes' numerics.
Thus when overlays came about, to allow conversion to ten-digit home NPA (and co-overlaid NPA) dialing, landline calls require 1+ before all NANP ten-digit calls.
But remember that NOT ALL of northern Illinois (or northwestern Indiana) are yet on a mandatory (1+)ten-digit dialing format, since the 708/464 overlay and the 312 & 773 / USA (872) overlay have NOT yet taken effect. NANPA hasn't even worked out any dates with the Illinois Commerce Commission or the telephone industry doing business in the Chicago metro area for such overlays and (1+)ten-digit mandatory.
I assume you know that wireless phones do NOT "have" to dial the 1+ in northern Illinois, New York City, or California overlays (or for that matter for ANY (at least US) originated NANP call regardless of US area code of origin or NANP area code of destination). Wireless origination can be "straight" ten-digits plus "SEND", or 1+ten-digits plus "SEND".
(NOTE that there COULD be some exceptions depending on the cell switch and/or service provider in question or if a Canadian or Caribbean destination area code -- it's possible that some cell providers require1+ for such ten-digit calls to Canada and/or Caribbean).
I don't think that the 1+ before ten-digit calls originated from landlines in 847/224, 815/779, 630/331, is going to become optional at any time soon. MAYBE at some point AFTER 1+ten goes mandatory from landline originated calls from 312 & 773 after 872 (USA) overlays, and464 overlays 708, will the I.C.C. and the various providers (at&t formerly sbc formerly Ameritech formerly Illinois Bell, and landline CLECs) consider allowing the 1+ to become optional on ten-digit calls originated from landlines in northern Illinois. And I don't know offhand if there are any office codes in northwestern Indiana 219 which "conflict" numerically with any area codes in the Illinois part of the Chicago metro area.
When all of Chicago Metro is fully overlaid and (1+)ten-digits is fully mandatory throughout the area for all NANP calls, then you should be asking this question. I don't think it is a good idea to have "too many" variations and exceptions on (landline) dialing when there is still some seven-digit dialing in place in the vicinity.