This is because of 2 different but linked problems
I've had a good number of occassions when someone said "but I sent a text to you" but I didn't get it..... PLUS my wife getting lots of charges for receiving texts when they should be free to receive...
I discovered the same reason for both
If you send a text (most phone systems) with an emoticon... a picture emoticon OR (some networks) a typed one eg

the sending network converts it (on a smartphone) to a graphic image and sends it as a picture, not text....
MOST networks charge for sending pictures but texts would be included in the phone package.... likewise SOME networks charge for RECEIVING pictures whereas texts are free.... OR the retrieval cost, from your mailbox/ message centre.... is much higher for a picture than text.
I don't get pictures on my phone as I don't use a smart-phone (I prefer an older "flip" phone)
SO an emoticon can cost sender and/or receiver much more... and an "emoticon'ed" message might never be delivered
I can't tell anybody what to do.... but I can give you something to think about.
I have found NOTHING, from networks, to tell you about this... but it can explain high charges (kids especially love emoticons) and non-delivery
malcolm