When an iPhone is booted up for the first time or freshly after a full restore, the phone serial number and IMEI are checked against a database in Apple to make sure it’s allowed to work with the SIM card inserted. This also activates push notifications to the iPhone.

iPhones that are locked will only activate when a SIM card from the providing carrier is inserted. Apple can recognize this from the SIM ID (ICCID) and the SIM IMSI. (This doesn’t have to be the iPhone’s original SIM card and it doesn’t have to be a SIM with active service – it just has to be a SIM that is valid for that iPhone.)

Activation opens access to all functions of the iPhone except accepting another carrier SIM card, which is controlled by the baseband (this runs the low-level phone functions).

What is hacktivation, and how can I hacktivate my device?

If an official SIM card is not available, an iPhone can be hacktivated to allow access to the iPhone functions and Springboard. You can hacktivate a device by jailbreaking it with redsn0w and/or creating custom firmware files using tools like sn0wbreeze or redsn0w on an iPhone 4, iPhone 3GS, or iPhone 3G. (There is no way to hacktivate an iPhone 4S, 5, 5s, or 5c, since they do not have bootrom exploits available.)

A hacktivated device has some downsides: push notifications, iMessage, and FaceTime won’t work. Your device will also drain battery and data while it tries to look for the certificates that allow a normal device to use those services.

How can I properly activate my hactivated  iPhone 4, iPhone 3GS, or iPhone 3G?

You may want to “un-hacktivate” your device, or in other words, properly activate it.

Official activation using redsn0w: If you can get access to a valid SIM card from the phone’s official carrier (it doesn’t matter whether the SIM card has service or not), put the SIM card in your device. Then open redsn0w, click “Jailbreak”, and click the “deactivate” checkbox that will be presented during the jailbreaking process. Open up iTunes, and it should properly activate your phone. You can then remove that SIM card.

Official activation using SAM: This is similar to the redsn0w method above – you’ll need to obtain a valid SIM from the phone’s official carrier, and put that SIM in your phone. Add the repository http://repo.bingner.com/ to Cydia, install SAM (Subscriber Artificial Module), choose “Revert Lockdownd to Stock”, and choose “deactivate”. Then plug your device into your computer, open iTunes, and it should properly activate your phone. You can then remove that SIM card.

Simulated activation using SAM: This method does not require having an official SIM card. You can use SAM (Subscriber Artificial Module) to obtain correct activation certificates for your phone, because SAM simulates an official SIM card ID (ICCID) and IMSI. See “How to use SAM to get proper activation of my iPhone?” for instructions.

Source: jailbreakqa