I can't text another iPhone?

I have an iPhone 5 with the ios7 update. When I go to text another iPhone it doesn't go through, even if the iMessage is off. Neither work! But I can text other phones! My sister is on my plan with the same phone and she can text iPhones? Is my phone just defective!

Try the following:

1) Restart your phone (switch it off, wait 10s, turn it back on)
2) Ensure iMessage is enabled

If it still doesn't work
3) It might be a carrier issue and check with your telephone provider ( you could try sending with your Wi-Fi to see if that works)

4) Try a restore from backup and see if that works (this is a tedious process but I really don't think you have to do this)