They were able to get the Burner close enough to start the cabin on fire. I have to believe they had a way to drop a cell phone safely and close enough for him to grab. Edit: Hell for that matter, and within 5 minutes of being presented with the problem- why not sent the EOD robot up to the door and drop it on the doorstep?
On the flip side, he allegedly and probably said some version of No Quarter in his manifesto thing.
Of course, given his training, and presumed knowledge of at least the basic tactics that would be used, how often this sort of thing has happened before.... wouldn't we all be screwed but say we knew it would happen if he turns out to have gotten away?