Callsign Sheltron is the callsign for a 33rd soldier in Spec Ops: The Line.
Description[]
In Chapter 14, a 33rd soldier at the beginning of the chapter will radio in with a panicked voice, saying: "Sheltron 1 to Tower, do you read?!?" Tower replies: "Copy, Sheltron. What's the situation?" Sheltron replies: "We need backup!" Tower replies: "What???" Sheltron replies: "Don't ask question! Just send backup!"
Basically Sheltron is just alerting the 33rd that Delta Squad have showed up at the front gate and are about to storm the final stronghold of the 33rd.
It is assumed this soldier is killed fairly early in the chapter, as it is never heard again throughout the level.