The research done at Bell Labs is the foundation of the information age, however, Bell Labs sowed the seeds that made the post-divestiture AT&T a doomed enterprise from the start - there is a reason they only lasted ~20 years from divestiture 'til they were bought by one of their former children, SBC.

AT&T provided for most of its history, the best quality telephone service in the world, at a comparable price to anyone else, anywhere.

There were structural issues with the AT&T monopoly however, for example cross subsidization - the true cost of services was often hidden because they would use optional services (like toll calling) to subsidize basic access, and business lines would cross subsidize residential service.

The level that AT&T fought foreign connections (aka, bring your own phone), probably hastened their demise, in the end, the very technologies that AT&T introduced would turn long distance from a high margin, to low margin business - the brass at AT&T had to know that, but they still pinned the future of their manufacturing business on that - a manufacturing business that had never had to work in a competitive environment, yet was now expected to - because of this and other factors divestiture was doomed to failure.

I'm a believer in utilities being a natural monopoly, but AT&T was an example of effective regulatory capture, it did not, and does not have to be this way, however it was.