Problems when texting 086 Thai Cell number

I am trying to text a cell phone in Thailand which is using an 086 Thai number. When the progress bar reaches about 95% it fails and I get the "Not Delivered" status.

However, I have texted other Thai numbers from my AT&T phone, and there are no issues. I have also been able to text the 086 Thai number from the T-Mobile network. This suggests that AT&T have problems (or restrictions?) texting the 086 cell phone numbers in Thailand?

Any ideas or insight into what I am observing?

Thanks,
Paul.