sierra2469alpha wrote:ansh_k - best just return it to whoever the provider is, explain the situation, ask them for a replacement. ENSURE they make a note on their system that you returned it to the provider. ENSURE you register your personal particulars with them, You don't want to be stuck with a SIM card that's registered to somebody else. Furthermore, it's actually illegal to purchase a SIM here in SIN as a foreigner without proper ID - something that other people around this forum should know. I'll probably get flamed big time for pointing this out to a moderator.
In theory, your advise is correct and this is absolutely what a good, upstanding citizen should do if they mistakenly purchase a second-hand SIM. You should definitely also go to the police, confess your crime and ask to get whacked with a rotan.
In practice, doing so would be just asking for trouble, expense or, at the very least, exposure to the turgid bureaucracy of the local operators. Given the world of pain that tends to ensue when you ask SingTel "can you sell me this phone for the price you're advertising", I can only imagine what would happen if you walk up and tell them that you want to re-register an illegally sold prepaid card...
So I'll stand by my suggestion: if you're here in Singapore for a few weeks, then just use the SIM and don't worry about it. And if you're here for a longer time... then throw away the prepaid and sign up for a legitimate, and much cheaper, postpaid account.