Sony Xperia Z tablet SMS issue

Hi all,

I have a Sony Xperia Z tablet (specifically the model SGP321) which I have on contract with MTN (specifically the contract: MTN Broadband 500 Topup package)

Now, I have a very irritating issue, I can receive SMS' on the device, but not send SMS'

I went into the MTN store in Somerset Mall tonight and the guy couldn't fix it - I'm sure he would've tried really hard, but considering I sat in the store for over an hour, I was gatvol and figured I might be able to fix it myself.

Now, apparently the problem is that the SMS centre number is wrong (Guy at the MTN store)

It's currently +27831000002 and it should be +27831000113.

In the Messages > Settings > General Settings > SMS centre number is unchangeable.

Any idea on how to fix this or has someone got any insight into this issue.

Thanks!