Plot summary
Special Counsel discovers that there is a threat to the Machine, which Finch and Reese are attempting to stop. The Machine gives a number belonging to Ernest Thornhill, a CEO who is buying several payphone companies in New York. Decima Technologies, Finch, Root, Shaw and Reese all attempt to reach Thornhill, but Finch discovers that Thornhill is an alias created by the Machine. Root and Finch enter Thornhill Enterprises and discover that the Machine will reboot to defend against the virus, once complete giving Admin access to who ever receives a call.
Reese and Shaw follow and encounter Greer and his men. Greer tells Reese that the laptop Reese had to recover with Kara Stanton from China was sent there by Finch. Finch knows which payphone the Machine is going to call and goes with Root to answer, followed by Shaw and Reese, where they encounter Greer's men. When the call comes, Root answers the phone, but Finch splices the call to a payphone near Reese so he too can answer the phone.
Credits
Adapted from Wikipedia contributors, Person of Interest season 2. HTML formatting and reference markers removed.