This is an expression commonly used to signal to another
party that either a) you no longer want to text them, or b) you intend on hanging up. This is a
smooth way to avoid answering or hearing things from other
people.
However, sometimes, it's true, and the person's phone really is about to die. This is rare.
(speaking on the phone)
Katie: So I've really been thinking about you a lot. I think I might love y-
Michael: Oh, shi*, sorry, my phone's about to die! -
click-
(texting)
Chelsea: Myfriend
Kyle said he really digs u. what did u think about him?
Sam: sry my phones about 2 die