It is only US models HTC themselves post RUUs for. Personally I'd try the Asian one first if that's a HK handset, just because of the radio bands. It is older, which should normally stop it installing, but S-Off should trump that.
As for S-OFF itself, if you are not the original owner it's possible someone else did that. But a few handsets do hit the streets already S-OFF, I don't think anyone knows why. I'd not worry about that, except to note that while it gives you more options it means you are less protected against installing something incompatible.
If the main problem is the phone app have you tried less drastic solutions such as clearing that app's cache, or a factory reset? The RUU will reset the phone anyway so you've nothing to lose by trying that. Though I'm not certain that even an RUU will fix all of this: a misbehaving patch on the screen might be a hardware problem. Do the speakers work normally apart from ringtones or notifications?
