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“Windows was available in Nebraska. This commercial is a parody of other commercials that, for legal reasons, had to exclude Nebraska. That line in this commercial is a wink and a nod to that.
So what was up with those commercials being parodied?
Prior to 1991, all 800 numbers were carrier-specific. Because of the mixed-up telephone system the US had after breakup of Bell, this meant that to call from one side of the country to another (such as on an 800 call), you may have to cross several carriers. This could get expensive.
To limit this, companies with 800 numbers got wise and centralized in the middle of the US. Due to its central location, Nebraska ended up the destination of many call centers for the, on average, cheaper 800 call rates. So companies moved their call centers to Nebraska for financial reasons.
But why did that make offers void? Because this 800 system could not successfully route in-state and out-of-state calls until 1981, when AT&T developed the tech to do so.
So throughout the 60s, 70s, and early 80s, the offers were void to Nebraska callers because they literally could not make the phone call.
As dumb as this sounds, remember: until the government stepped in in 1983, phone companies wouldn’t even allow consumers to own their own phone!
As an aside, there are still a lot of call centers in Nebraska, but the restirctions on 800 numbers being loosened has created a long, slow exodus of such jobs.
Refs: https://www.ringcentral.com/blog/the-history-of-800-numbers/...