My iphone 5 will not text one number

Have seen previous threads on this issue but none have resolved this matter for me.

There is one number that I cannot text, the other p contact a Samsung GT-S3350(Tesco) mine is on Vodafone. I can receive texts and calls and can call the number, but texts fail to send and are all marked as Not Delivered.

I have turned both phones on and off, turned on/off iMessage, 3G, data roaming and have reset network settings. I have also deleted the contact from my phone and the other phone.

My wife has an iphone 5 and doesnt have any problems texting the number.

Can anyone help?

Thanks!