The internet is another story. That was designed by DARPA to survive nuclear war.
They have full cell coverage in Mogadishu in the middle of battles. And they have full coverage across Afghanistan.
The govt can shut down some exchanges, but it will be spotty at best.
Cell systems primarily enter the phone system Central Office (CO) of your local community in order to access the larger network connectivity. Some cell systems are wired to talk to each other but usually even then they connect via the CO. So, taking down the cell network is as simple as turning off the network switches at the CO. Bye-bye cell system. A CO usually serves up to 50,000 subscribers then another CO is built. So, doing a little math means a town/city of 250,000 might have about 5 COs and up to 15 if need be, depending on age of the phone network and terrain. So, 15 COs to turn off, gimme 5 minutes, those switches can be turned off remotely.
As far as the Internet, it talks like a web (gosh, go figure). No one computer controls much of anything and every computer can talk to any computer it can reach to pass a message. Eventually, maybe a bazillion connections later, the message actually reaches the intended computer. To take down the Internet would require taking down all major communications links. MCI used to control 90% of all Internet traffic, now bought by Verizon, but it still is the major hub. What is interesting is that much of their communications network is microwave relay links. It’s how MCI started in the first place in the 1960’s. There isn’t a simple link to kill the Internet, or a few massive pipes. The Internet still relies on older microwaves, copper cables, and older digital phone networks. Sure, much of it is carried on large pipes, but not all of it.
Taking down all that interconnectivity can be done but it would essentially blackout just about all communication links including those needed by FMSL (Federal, Military, State, Local) governments. It would also take days just to coordinate it all. Sure, there has been talk of an off switch to it all, controlled by the whitehut, but that simply has not happened. The feds could just give the order that all commercial systems must shut down but even then the FMSL would suffer, too. I also highly doubt that order would be followed.
P.S. Much of the fiber laid in the 1990’s lays dormant. Much of it connects to nothing having been laid into areas with anticipated connections to other fiber networks but those networks were never laid. There are those little amplifier buildings all over the place with weeds growing around them and stacks of fiber and other equipment just rusting and sitting there. Billions and billions of investor dollars from the DotBomb era lay wasting in the ground.