Galaxy S5 :: Cannot Send Text Messages (but Can Receive) Even After Factory Reset

My Galaxy S5 was bought new and boxed on August 6th. After 5 weeks it started playing up as the ringtone would play over and over with every new notification (FB, Twitter etc) meaning I had to turn the phone off. A reboot and Zedge app delete sorted this out however now I find I cannot sent text messages but I can receive them. I can also post on all social media but not send a text.

I have twice restored the phone to factory settings which solves the problem for about a week, then it returns. I'm on 4G with Unlimited Texts and I'm not over my allowance. To date I have tried the following which didn't work....

* Switched the phone off then back on
* Installed the latest software
* Switched to Flight Mode then back again
* Checked the text message centre number (+447782000800) is correct
* Restored phone to factory settings (twice)
* Removed WhatsApp
* Removed all music and video media

I managed to get my text service up and running (so far) by clearing the message cache. Here is how to do it on Samsung S5 APPS - SETTINGS - scroll down to APPLICATIONS - APPLICATION MANAGER - scroll across the screen to the left to the 'ALL' menu - scroll down to MESSAGES - CLEAR CACHE. Finally re-start your phone to allow changes to take effect.