Problem is, what happens in cable doesn't stay in cable, if you'll forgive the misquote. Especially regarding the phone system, where most long distance is bounced off satellites.
Not to mention that it'll be digitized, mixed with a thousand other calls, placed into the internet framework, bounced through 4-6 different company's servers, travelled via fiberoptic cable, satellite, and underwater cable before being separated from the datastream and recontituted as your phone call.
If at any point, the players in this dance decide that they aren't going to play anymore... our communications suffer or, in the worst case, stop.
Let's not forget that there is also our military DSN phone system that runs on standard corporate phone lines. That requires our telecommunication companies prioritize these lines above all others. The FCC makes sure they actually do so, by providing all the phone number information to the companies and ensuring they actually comply.
And it was critical for our military immediately after 9/11, so that our military units could coordinate with each other. That the President and the Pentagon could actually pick up a phone and call their component commanders to direct subordinate units. Not to mention that it was critical for individual units, as they had to conduct telephonic recalls to get their personnel in ASAP.
During 9/11, our phone system was overloaded with civilians calling friends and family in the affected areas. Had there been no FCC and no oversight of our DSN system, our military would have been paralyzed for some time.