If someone uses Sing Tel's card in Malaysia, how can they send a message to China?

You can call the customer service number of Singapore Telecom to ask. They have Chinese customer service. I usually send text messages in China and then reply in Singapore. But ... it seems that text messages are expensive. You might as well make an international call directly. On weekends and leisure time, international long-distance charges are very cheap. Or you can buy that callback card. Or, if the mobile phone is a smart phone, you can also install VoIP software, which is cheaper.