After Ma-Ma's clan machine guns Peach Trees so bad that some bullets penetrate the concrete and exit the building, Dredd blows a hole in the wall so he and Rookie Anderson get out of the building, on a high floor (50-something?), near a skating rink.
Even though they were aware that the blast shields had cut off their comms, it takes them a few precious minutes to realize that now comms should work, but eventually Dredd does call for backup and mentions they've been under fire and can't maintain their position.
Why does the backup only include 2 judges apparently arriving on foot? Why no aircraft, given their precarious position up there, or at least a van? This was the famous Judge Dredd asking for backup, not any random judge, so he was both important, and in a dire enough situation to ask for backup (in the beginning of the movie, the Justice Hall operator asks him, "Do you require backup?" and proudly answers "No"; but not this time).