Just use the 009 prefix as this will yield the lowest cost. Example: 00912125551212. 009countrycodecity/areacodenumber. Quality is fine.
It looks like calls to Singapore and Canada will be 5 baht/min (exclusive of 7% VAT). International SMSes are 10 baht each (I think). Local calls will vary by promotion/plan. Figure 2 ~ 4 baht/min.
Data (GPRS/EDGE) typically runs 1 baht/min unless you subscribe to a plan/promotion.
International rates are fixed so no difference by carrier.
No charge for incoming calls or SMSes.