Many people seem to consider the mobile phone being unidentified is suspicious. As someone who lives in Asia I can say that most of the phones here are of the same type. A sim card is bought at a 7/11 or any corner shop. It is prepaid and can be topped up. Postpaid phoness are a much more complicated process as they involve credit checks etc.
In theory ID is needed to buy a phone but almost without exception the clerk at 7/11 will make an error in transcribing the ID munber. This may of course be an accident but it happens so often that I think an understanding of privacy needs is assumed or a dislike of being regimented.
As it is so easy to get a phone number may change them often. In fact last week I left my phone home and had to buy a sim myself so I could use other people's phones to ring home. It took me one minute.