The Messages app, included with iOS 7, allows you to send text messages to any device that accepts text messages. Messages attempts to send text messages through the iMessages system first. If that service doesn't succeed, then Messages attempts to send a message as an SMS. Macs running OS X Mavericks can also send and receive iMessages provided the user registered with iMessages. Messages sent using iMessage display in blue as you type the message, while SMS messages show up as green text.
Differences Between iMessage and SMS
The iMessage and SMS service provide a similar service that provides a way to send text messages to recipients. The iMessage service provides a way to avoid the charges imposed by your cellular data provider for text messaging services. It does this by sending messages over a Wi-Fi or cellular data connection to other devices registered with iMessage. This includes all iOS 7 devices and Macs running OS X Mountain Lion or higher. If you use a cellular data connection you can still get charged for bandwidth used, but you won't get a fee for sending a text message. The SMS service fills in the gaps with iMessage by providing a way to send a message to any device that supports SMS messaging.
Enable SMS Support
Check to see that SMS support is enabled on your iOS device. Tap the "Settings" app, select "Messages" and set the "Send as SMS" toggle switch to the on position. If you don't see this option, you need to first set the "iMessage" toggle switch to the on position. If you want to enable group messaging as well, then set the "MMS Messaging" switch to the on position. Setting the configuration properly ensures that you activated the SMS messaging service.
Software and Carrier Settings
Outdated iOS software and carrier settings can affect your ability to send text messages. Tap the "Settings" app, choose "General" and select the "Software Update" option. Wait while your iPhone checks for updates and follow the prompts to install any new updates. Go back to the General screen and select the "About" option. If you get a prompt to update your carrier settings, confirm and install any available updates. To install updates, you need to select the "Update" or "OK" button. The "OK" button displays when your carrier releases a mandatory update.
Apple Services System Status
Sometimes the servers used to transmit messages go down. While this shouldn't affect your ability to send an SMS message, it could cause issues with the iMessage app itself. Check the server status and make sure that the iMessages service shows up as green before attempting other troubleshooting steps (link in Resources). You should also contact your cellular provider to verify that you have text messaging services enabled on your data plan.
Check to ensure that you have Airplane Mode turned off in the Settings app. Airplane Mode prevents you from sending and receiving text messages as well as phone calls. After checking Airplane Mode, restart your iPhone to see if that corrects the issue. In cases where you still can't send a text message, access the "Settings" app, select "General" and choose the "Reset" option. Select the option to "Reset Network Settings." Make sure you don't accidentally select "Erase All Content and Settings" as this causes you to lose data. Finally, if you still can't send text messages, try removing the SIM card from your device and replacing it, then restarting your iPhone.