I used to work in premium SMS, but in the UK and EU regions, so only have recommendations from my experience there.
mBlox - The World's Largest Mobile Transaction Network were always good at dirt-cheap MT, and their MO wasn't so bad.
The best I ever worked with, though, were
Netsize - The global mobile commerce enabler: Mobile Messaging, Mobile Payment, Mobile Content Management and Mobile Internet portals.. They kicked serious arse, decent platform, lots of checks in places with the various operators, and always sent out status emails when (not if... when) particular operators were having issues doing their day-to-day business.