I dunno if this is the phone, but when I get messages sent to me, I never get them. Recently I have been told that they sent something and I never replied, but I never got it. Most of my friends have Verizon and one has TMobile and I get the problem with both.
I've found the main problem with people not having their messages recieved or actually sent to the person that they are trying to send them to is that you have to put in the area code or the message never gets there. My friend was having the same problem so I investigated it. I don't know if this is Sprint specific but I know it fixed it.