Could be because it's an SE phone. :P
I've had his happen with the Vivaz Pros running Symbian. But at a much larger scale. It not only texted, but would dial out as well. Could never figure out if it was a hardware or software OS defect. Happened with many Vivaz owners.
Why am I bringing this up? Because it's an SE phone and my previous experiences with them have not been pleasant ones. But it's their mobile division I have very little faith in.