@ashyyk the way I see it, it means what it means...
"Celcom will activate roaming services with no additional charges for you. If you do not wish to have this service, please reply 'NO' to 28889 within the next 3 days."
You may refer this link https://www.celcom.com.my/personal/roaming/postpaid for rate of international roaming. You also can call our customer service at 1111 (from Celcom no) or 0196011111 (from another telco) to get info about international roaming.