I do remember hearing that one of the signs you’re in a dream is that your phone doesn’t work properly

It's one of the "reality tests" of lucid dreamers. You make a mental note of how you know your phone to behave and if you discover that not to be the case then that acts as an "alarm" that you're sleeping.

There are other tests as well. Light switches never turn on the lights. Text changes when you look away and then read it again. You can try pushing your hand thru the wall. If you jump you'll jump really high or may even start flying.

The point of these tests is to do them daily even when you "know" you're awake so that the habit carries over to the dream world. However when this eventually happens either of two things is the most likely to happen: you freak out and wake up or you rationalize it to youself somehow. For example: "Holy shit I'm flying, this is a dream! Oh never mind it was just a crane that's lifting me" Speaking from experience.

Also a word of caution. It's not recommended to look into mirrors when you're lucid dreaming. You'll see something other than yourself and this can be really uncomfortable. Also if you realize you're dreaming and start to freak out but don't wake up there's a good chance it turns into a nightmare. The way out of it is to not be afraid, for example go hug the monster instead of running away but this is easier said than done.