@mononoaware >just register a SIM card to a dead person or some other front
Yes, you can do that, but in mobile plans, the mobile provider listens records the calls and the SMS's you send and any slip-up means that the mobile provider can easily learn who's using the SIM and that the provided information was faked.

A data-only SIM usually doesn't need any information except for a delivery address and the mobile provider cannot see the contents of the calls you make or SMS's you send if you use proper encryption for SIP and the transport.