When Dwight reveals to the rest of the office that it was only a fire drill, he says there is no fire, but he did actually start a fire in a garbage can. No one was seen putting it out. Response: Dwight just meant that the building was not on fire.
At 00:01:01, you can hear one of the phones ringing as smoke starts coming in under the door. At 00:01:27 in, Pam says the phones are dead. Dwight is at his desk at this time, so it would've been impossible for him to cut the phones in 26 seconds.