I bought my wife a new iphone 5s for Mother's Day. She of course loves it, but informed me that she was unable to message me and kept getting the following message:
Message sent using invalid number of digits. Please resend using 10 digit number or valid short code. Msg 2114.
Our carrier is Sprint and there seems to be an issue with trying to send a text message using only seven digits. In researching the problem I found that this problem was not exclusive to the iPhone 5s, but the fix seems to involve similar setting changes. For the most part you need to delete the person you're having problems texting from your contact list and delete any text messages you attempted to send. Also delete the error messages you received for each attempt. After you've done that reboot the phone and try sending that person a text. For my wife's phone I went straight to messages and composed a new message with my number.
You'll need to try something similar on other phones, but here are the exact directions on an iPhone 5s:
Open Contacts -> select the contact -> select edit -> scroll all the way to the bottom and delete contact (delete multiple entries of the same phone number, my wife had five).
Open the Messages app -> select Edit -> select the red circle and then select delete. Do this for both the person and the messages you received.
Open the Settings app -> scroll down and select Messages -> turn off messages, by selecting the switch, and any other options turned on.
Turn off your phone and then turn it back on.
Go straight to messages and compose a new message and put in the phone number of the person you're trying to text. Send and that person should receive the text message. Add the person to your contacts and go back into message settings to turn on any other options you want on that you turned off.