The number to call is +66 2356 1111 (you may already have been calling this).
Press 9 for english, and press 4 for international ticketing/reservation. Just tried it from NZ and it worked OK.
There is a short burst in Eng on the new service to Koh Samui...but you can just press 4 a few seconds after pressing 9. Just be sure that the phone has 9 where you think it is.....really!