The rules are being enforced by Thai National Broadcasting and Telecommunications Commission (NBTC), and anyone that has a prepaid SIM card in Thailand must provide their personal details before July 31st. This does not apply to those people on contracts, as their details are already captured.
The new rules start today on February 1st, after which users will have 6 months to provide their personal details (name, ID, and of course the phone number in question). Failure to do so means that you will be unable to make outgoing calls, text or use data by the end of the deadline. However, you will still be able to receive calls.