Here's how to change the message centre number on your Windows Phone 8S by HTC.
- From the homescreen, tap Messaging
- Tap the three dots in the bottom righthand corner of the screen
- Tap Settings
- Tap SMS centre number
- If you're an Orange customer ensure +447973100973 is showing. If you're T-Mobile, the number is +447958879879
- Press Save
Then just tap the Windows key to return to the homescreen, and you're done.