Straight forward: there are no anonymous SIM cards. This is technically impossible. All of them are just a big time scam that take advantage on lack of knowledge from regular people. And nothing more.
Fact: there is no SIM card without an IMSI.
Fact: there is no connection to a cell tower without IMSI being used for connection purposes.
Fact: even data only SIM cards have an IMSI assigned by the manufacturer.
Fact: there are so called IMSI catchers, designed especially for call/SMS interception based – as its name says – on IMSI.
Fact: if you can call whatever number or you can receive calls, that means your phone is connected to a cell tower, by using voice/data channels.
Fact: once connected to a cell tower, almost ANY cell phone location can be (and it is) tracked by various technologies and systems, taking advantage on mobile network weaknesses or mobile network nodes (SS7).
Fact: once connected to a cell tower, any phone call can be (and it is) intercepted, no matter if voice call is routed on standard voice channels (regular voice call) or on data channels (as Skype, IM, WhatsApp, etc.).
Fact: not the SIM card is choosing the cell tower to connect on it, but the phone. This is the way that all mobile networks are designed (no matter if 2G, 3G, 4G, etc.), a SIM card being only used to identify a certain subscriber.
Fact: the phone number is not stored on the SIM card. Phone number is stored on mobile network servers (HLR/VLR) and cannot be altered/changed directly from the phone/SIM. A phone number can be altered/changed ONLY by using data connection and 3rd party servers. Some particular “russian SIM cards” are using standard voice channels that still route the call trough some russian server, where in fact take place voice changing, and only then the call is routed to call recipient.
Fact: ANY SIM card is encrypted by default, using comp128 algorithm. There is no other encryption that a SIM card support. This is meant for anti-cloning purposes. Some early comp128 versions has been compromised, old SIM cards (until 2012) being easy to clone.
Fact: ANY regular phone call on ANY mobile network (no matter if 2G, 3G, 4G, etc.) is encrypted by default. Otherwise, anyone with a radio receiver can intercept that call. A SIM card CANNOT ad another layer of encryption on top of existing one, nor additional encryption.
Fact: ANY SIM card is trackable and any phone calls and SMSs done with a SIM card can be intercepted.
Fact: IMSI is not the same with phone number, nor with ICCID. IMSI is stored on SIM, since phone number is stored on carrier servers.
Fact: IMSI is not printed on SIM, but ICCID.
Fact: anyone can figure out its own SIM card IMSI, by using freely available apps (both on Google Play and App Store). If you average Joe can do this, then law enforcement or hackers can do that remotely, over the air.
Fact: IMSI change is possible by sending special requests to SIM issuer (the mobile network that issued that particular SIM). The request cannot be sent directly by the SIM user but by other company, in his name (used for example when porting a phone number). Changing IMSI this way is not a standard procedure, despite the fact that IMSI change is mentioned on GSMA and 3GPP procedures. Fraudulent MVNO companies (mostly russian) are taking advantage on this procedure, enforcing the law because the MNO doesn’t care, changing SIMs IMSI based on user direct request.
Started back in 2014, anonymous SIM card scam refers to a few types of SIM cards that are being sold to people without a decent knowledge regarding mobile networks: