A number of users have complained that after updating their iPhone to iOS 7 they have encountered a number of issues with iMessage.
Users are reporting unsent and unreceived messages and some sent as SMS when they should have been iMessages. Others have complained that iMessages are not being converted to SMS to send to users on difference devices.
Members of Apple’s Support Communities forum have posted threads detailing a solution to the problem, with many reporting success with the method.
Disable iMessage in Settings -> Messages, reset the iPhone's Network Settings under Settings -> General -> Reset, then reenable iMessage.
This is not the first issue Apple’s latest iOS has encountered. When it first launched on September 18th many had trouble upgrading, with some receiving error messages and seeing incredibly slow download speeds.
A security flaw was also discovered which allowed anybody to bypass a passcode-locked iPhone by using iOS 7’s new ‘Control Centre’ from the lock screen.