I’m pretty sure you’ve got it wrong.
Firstly, AT&T was already a monopoly by 1913. The Kingsbury Commitment[1], which is what I’m guessing you’re referring to, was in essence an agreement between AT&T and the government by which the government would tolerate the monopolistic status of the company in exchange for some minor concessions. While this didn’t hinder AT&T much, it also didn’t help them.
The way AT&T became a monopoly to begin with is quite simple, and basically a textbook example of how a natural monopoly arises: Early and continued investment in a business which has large fixed cost requirements (that is the cost to build out infrastructure), but tiny marginal cost (the cost of serving one more customer). Unchecked, this eventually builds up to an insurmountable barrier of entry for new players. At the same time incumbent players are highly incentivized to merge or acquire each other, because the value of their combined networks is greater than the sum of them.
Without government intervention, AT&T would have stayed a monopoly for as long as its infrastructure remained relevant. Which it still largely is.
[1] UNNATURAL MONOPOLY: CRITICAL MOMENTS IN THE DEVELOPMENT OF THE BELL SYSTEM MONOPOLY https://object.cato.org/sites/cato.org/files/serials/files/c...
So, in your mind what caused AT&T to become a monopoly for so long?