Why does my iPhone 4 says Not Delivered on some of my texts?

When I go to send a text it will say Not Delivered! But yet the other person usually does receive it so then it sends twice cause I usually hit retry. Does anyone know why it does this? And it doesn't happen all the time, just occasionally.

It looks like you are having trouble sending text messages from your iPhone. I would like to point you to a resource that may help you to get a more 'device specific' answers. Please refer to the following:

http://www.apple.com/...rt/iphone/

You may also consider reaching out to your mobile carrier.