As soon as iPhone Dev-Team failed to produce Ultrasn0w unlock for iPhone 4 2.10.04, 3.10.01 baseband, a Chinese hacker team called Gevey started to look into the issue and found a working solution which can actually unlock iPhone 4 on said the basebands. Before i may go into pros and cons of Gevey's iPhone 4 unlock, i would like to clarify what is Gevey SIM hack and how it works to unlock your iPhone 4?

What is Gevey SIM Hack?

With Gevey SIM hack, you can unlock iPhone 4 01.59.00, 02.10.04 & 03.10.01 baseband. Unlike Ultrasn0w unlock, Gevey SIM hack is not a software based hack as it requires you to buy a SIM interposer which sits electrically between the SIM card and baseband hardware to perform a classical MITM. In order to stay unlocked, you ALWAYS need to keep that SIM interposer in-between your SIM card and iPhone baseband hardware.

While your iPhone is unlocked with Gevey's SIM interposer, if it loses power due to low battery or loses reception due to poor signal coverage, you have to perform the whole unlock procedure again.

How Gevey SIM Interposer Work to Unlock iPhone 4?

Although SIM card holds many different types of information but the part which involves carrier unlock is the IMSI number. IMSI (International Mobile Subscriber Identity) is a unique number that corresponds to your account in the mobile carrier's database. Every IMSI number consists of a MCC (Mobile Country Code) and MNC (Mobile Network Code).

Example IMSI number: 302 720 123456789

The above IMSI indicate the SIM is from Canada (302) Rogers Wireless (720). When there was no software unlock available, the iPhone baseband versions used to check IMSI twice following a restart but later Apple made the authentication procedure mode difficult to crack by introducing a 4-byte TMSI (Temporary Mobile Subscriber Identity) which now identify each handset before IMSI is sent and processed.

So how Gevey SIM still manages to trick the TMSI? It actually makes use of the emergency number 112 to get the TMSI for your connection. When the network issues TMSI for your connection, Gevey's SIM interposer finds an acceptable MCC/MNC combination by rapidly cycling a list of IMSI. To prevent your baseband from detecting the fake IMSI, toggling the flight mode for a second is enough. This spoofing of IMSI and ICCID makes your carrier think that your iPhone 4 is factory unlocked.

Without any doubt, spoofing of IMSI and ICCID using a legitimate emergency number is unethical or downright illegal. Also keep in mind that your network can request your IMEI during the emergency call and can ban your account for using false information and tampering with your SIM card.

Check out the video demo of iPhone 4 unlocking with Gevey's SIM interposer [via Singularity]:

Things You Need to Know Before Buying Gevey SIM:

1. It works if your network handles 112 calls properly according to the GSM standard. So before buying Gevey SIM, first step is to dial 112 from your phone and check the response.
2. If your network validates your IMSI on every incoming call, do not think of buying Gevey SIM.
3. Even if your network do not validates your IMSI on every incoming call, you still have to revalidate your IMSI if your iPhone loses power, loses the reception or move to another PLMN. In all of these cases, your TMSI expires and has to be obtained again.
4. Not all 3G network will work, for example you will not see T-mobile with 3G service on the iPhone but you will see it working with edge.
5. SIM interposer will not harm your iPhone hardware but it can get your account banned from your network provider if they knew that you have spoofed your IMSI and ICCID. Its only in China that GSM companies know this loophole but still don't bother to close it.
6. The only way to permanently unlock your iPhone is through NCK.

UPDATE#1: Gevey SIM Can Unlock iPhone 4 04.10.01, 03.10.01 & 02.10.04 Baseband [Videos]
UPDATE#2: How to Use Gevey Turbo SIM to Unlock iPhone 4 on iOS 4.1/4.2.1/4.3.3? [Guide]
UPDATE#3: Get Gevey SIM to Unlock iPhone 4 for U.S. $22.80 [3% Discount for Jaxov Fans]
UPDATE#4: Gevey SIM Pro: Faster IC, No Need to Cut SIM, Support Multiple Carriers
UPDATE#5: Gevey Pro vs Gevey SIM [Hands-on]