I have moved this thread into the One X forum, so that those who use this phone can post their advice in this thread.

 

I can only comment on my AT&T One X(don't think this feature is different with the international One X as it's part of Sense). When a call comes in if you decline a moment later you should get the choice of text responses (already typed) on the screen, and if you choose one the caller then receives a text).
This has worked every time I have declined a call.