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.
(speakingon 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
I was supposed to call my gf but my phone was trippin'. My boss asked why I didn't show up at work and didn't bother to call or text and I told him my phone was trippin'.
1. When you're on the phone and you really just want to get off and you say: "My phone is dying, gotta let you go".
2. When the girl you laid on Thursday calls you on Saturday to "see how you're doing" and talks your ear off, you say: "My phone is dying, gotta let you go".
3. My phone didnt really die, I was just done talking to you.
Girl: "So I met up with Cindy on Saturday, I had not seen her since college. I think she's gained a little bit of weight since then..."
Guy: "Hey great to know you met up with Cindy. My phone's dying I gotta let you go."
Guy afterwards: calls the girl that he met on Friday.