Why does my Iphone keep saying not delivered?

So I'm texting 2 friends. One of them it won't send my text messages to even though I have service, yet it'll send messages to my other friend. I tried to send a text to some bogus phone number with not even enough numbers and it said it went through! I'm tired of trying to text them over in over just in case they are going through and I'm just not getting their replies. Why would it keep saying not delivered for some text messages to the same person but not the other person. Could it be their provider?

Their cell phone is either off not in range or the phones been stolen by some aliens and they are testing it to see what it does so they put it into airplane mode so they woulldnt be interupted… I'm just sayin *** happens.

That is possible that it is some sort of hiccup with their provider, but more likely it is your friend's phone. I had a friend have this happen when i texted him with his old phone and when he replaced it the problem went away.