That's rough. I was in a workplace where we forbid cell phone use, and issues like this came up.
Be honest, state that's why you were let go. You were let go for violating company policy due to cell phone use. Then in interviews, go into more detail why. But admittedly, you need to choose when to use your cell phone. If a child is sick, it makes sense you would do it. But what other reasons could you have done it? Could it have waited? I understand children are important, and so do employers, but I've had employees take every call from their kids and most of them being unimportant and could have waited.