Here's a tip for all users of the "One-2-Call" cards. When you buy it, get the sales person to call the call center and switch the default language from Thai to English. I did this and always get the prompts in English when I top up the card.

Standard rate for a new "One-2-Call" sim card is 400 baht (with 300 baht talktime included, valid for 30 days) but if you inquire at the Telewiz chain of handphone shops found all over Bangkok, you can get pretty good deals. I paid 1500 baht for a new sim card with 3000 baht value on it, valid for 6 months. These promotions come and go. Top up cards are available in denominations of 300 and 500 baht, which add 30 and 50 days respectively to the validity of the card.

I've never used DTAC, but Orange is a cheaper alternative - calls are 2.5 baht per minute as opposed to 4 baht per minute for "One-2-Call" but the coverage of the latter is better in the countryside.

FYI, I saw a newspaper ad recently saying that you can activate international roaming on the "One-2-Call" card by calling the call center, but personally I don't think its worth it.

Also, I think DTAC uses GSM 1800 only. And receiving of SMS from the internet or any other telco operator is not dependent on whether its GSM 900 or 1800, but rather its dependent on whether the SMS gateway of the receiving operator is configured to accept SMSes from those sources. AFAIK, as a "One-2-Call" user, I can't receive SMS from ICQ.