Tasked with an S.O.S. message from "Defense", you complete this objective in "Outpost". Enter the first Nazi controlled building (the one where a Demon snatches a fleeing Nazi in a preset condition) and battle through it.

Get yourself on the upper level of that building. On the upper story, there is a hallway with three tripwire mines (and two Demons who are easily exploited for the Demon killer achievement). Navigate through the mines and check the right room at the end of the corridor. There is a ramp leading to the rooftop.

On the rooftop, you should see a pre-set corpse with a Kalashnikov. The communications apparatus is on the corner of this rooftop.