Dear dev
Plausibility police here.... This scene:
Tells me that you don't understand the concept of a Faraday Cage or, as the US Military Industrial Complex would refer to it, "Tempest Hardened".
If you want to see something that Hollywood actually got right for a change (IIRC), watch the Kevin Costner movie Postman, or better yet, read the book. Back in the 90's when USPS was automating the mail before the right wing grifters in a certain party began their sabotage, it was a popular favorite among the MDAS (Mail Directory Automation Specialist) community.
MC gets off the phone with his favorite lawyer from inside the cage. Then he grouses about not having the interwebs.
TLDR answer is that MC wouldn't be able to do squat with his phone short of playing locally based apps that don't need to go online. That grounded fine mesh of wire and metallic paint means that nothing RF will be able to do jack shit with the outside world. It keeps the electronics from getting fried from the nuclear or whatever apocolypse that happens a la the Postman.
The whole setup for this room IIRC never once mentions that this room has a hard connection to the outside world with that being straddled by a firewall. This is the only way that this could happen, but I would argue that the 4g/5g radio used by the cell function would still be dead while an internally located wi-fi server connection might be allowed to have the phone poke through a couple of ports opened on the firewall. Even this normally wouldn't be a thing unless the hard line is actually a fiber connection drilled through the cage because of the whole EMP scenario, but I would allow it here because the gang is only interested in filtering the connection activity.
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If you are interested in this shit and want to geek out on the beach while you still can with a good book, get Neil Stephenson's book Cryptonomicon. WARNING: It's a long read. Think War and Peace.