A guaranteed monopoly lasting eighty years or so isn't a government subsidy?
It's not clear, and Cato's document doesn't explain, how anyone outside of major cities would have been able to get telephone service in the absence of monopoly guarantees. Universal service would have taken decades otherwise.
There is some hand-waving in the document comparing telephone service, which required expensive and widely distributed equipment, easements and infrastructure, with radio and TV (which itself was regulated out the wazoo). This comparison doesn't survive even momentary scrutiny.
The Bell System was the envy and standard of the world, and I don't see how it could have been anywhere near as successful at meeting the needs of the entire US population without those initial monopoly guarantees. Contrast the telephone system's universal service mandate with privately-operated electrical distribution networks: sizable chunks of New York City and Boston were still using DC power until the 1960s.
I think the biggest problem with the AT&T monopoly was that they were allowed to milk it for decades longer than was good for anyone, including the AT&T companies themselves. The company should have been broken up the minute they showed a net profit.
Even that opinion is debatable, though -- we didn't get the first laser from General Electric, or the first transistor from Hewlett Packard. Both these inventions and countless more came from a company that, by government fiat, could engage in long-term research beyond that needed to show an immediate profit.
So, yeah... that Cato position paper is pretty goofy, IMHO, and it's one of several that will be on my mind when it's time to re-up my own membership.