What does it mean to be unlocked? What sort of locks are there? How is that different from Blocked? What is a Barred or Banned or Blacklisted cell phone? That is what this guide will look at.

Blacklisted -

If you phone is lost or stolen you can report the International Mobile Equipment Identifier (IMEI) to your carrier. They will broadcast that number throughout their network and make it impossible for the phone to connect to make a call. This known as blacklisting the phone. This is sometimes called a barred or banned  device. In some areas the various carriers share the IMEI numbers of lost and stolen devices making it impossible to use the device on any carrier. In some countries this sharing is a requirement of law.

A blacklisted phone should be returned to the owner. This can be done by dropping it off to the carrier branded on the phone. Then it is up to them to return it to the subscriber. You can check with your carrier prior to purchase to see if a phone will activate on that carrier. You will need the IMEI from the seller, then you are a phone call away from knowing if you are about to buy a lost or stolen lightweight desk ornament.

Blocked Cell Phone -

If there is an outstanding balance on the bill some carriers will block the device from being used until the bill is paid. Some carriers tie the balance to the device. If you purchase a blocked device you will need to pay the balance to get it running again, if they will let you. Some carriers block the device forever and some for six months. A call  to your carrier to check out  the IMEI or Equipment Serial Number (ESN) will allow you to avoid purchasing a worthless device.

Blocked SIM -

A Subscriber Identity Module (SIM) identifies the subscriber to the network through the International Mobile Subscriber Identity (IMSI). This allows you to move your SIM from device to device and have your carrier recognize who you are.

You can set your device to require a Personal Identity Number (PIN) to access the SIM. (see Security Lock below) If you enter the PIN incorrectly too many times the SIM will lock and you will need to get a Personal Unlock Key (PUK) from your carrier to gain access to the SIM. Enter the PUK incorrectly too many times and you will end up with the need for a new SIM. Messages vary by manufacturer as shown:

Make                            PUK Required                                              New SIM Required
Alcatel                     3 Bad Codes! PUK Code                                         Please call operator
Motorola                  Blocked                                                                            Blocked - See Supplier
NEC                        This SIM is blocked                                                 Unusable SIM
Nokia                      Simcard blocked - Enter PUK                                     Simcard rejected
Philips                     Enter PUK                                                            PIN blocked (May say SIM blocked
Samsung                 Please enter PUK Code                                           Contact service provider
Sharp Pin                Blocked Enter PUK                                               PUK Blocked
Sony Ericcson          Pin blocked - Unblock?                                             Blocked - Contact your Card Provider

Locked -

Service Provider Lock:

Most GSM devices are sold to be used with on a specific GSM carrier such as T-Mobile, Vodafone, Cingular, Rogers, Fido, TiM, etc. The device will accept only a SIM from the GSM carrier that sold the device. This is known as a Service Provider Lock (SP lock). You will know if you have a SP locked device if you receive one of a variety of message at startup such as
"Subsidy Password",
"Phone Restricted", 
"Insert correct SIM card",
etc.
and the phone does not budge until you enter the code or put the appropriately branded SIM in place. Entering the unlock code incorrectly too many times will result in the condition known as hard lock. This requires the use of a piece of hardware to remove the Service Provider Lock.The hardware can be a stand-alone item  known as a clip or it may be a computer with unlocking software and a cable connecting your device to the computer. The SP Lock is also, less precisely, called a SIM lock.

Security Lock:

The end user of a device can provide that a password be entered to allow access to the SIM. Since the IMSI information is on the SIM this effectively prevents unauthorised connection to the network. This properly is called a Security Lock. The password is known as the Personal Identity Number (PIN). This is also known, less precisely,  as a SIM lock.

Network Lock:

Devices may be locked to a particular network. For example 310-260 is a network code for T-Mobile in the USA. In Germany the T-Mobile network code is 206-01. The network lock would allow T-Mobile to prevent a user from using the T-Mobile USA device on the T-Mobile Germany network and vice-versa. Related to this is the Country Lock which limits a device to networks within a specific country. The first three digits of the network code represents the country of the network. A Master Unlock Code will remove all of the restrictions from a device.

Unlocked -

Hopefully when you see this in the title or body of an ad for a cell phone the seller is referring to the Service Provider Lock or the Network Lock which are the most commonly engaged locks.  If the Service Provider and the Network locks are removed then you may use any SIM in the device. This is because Country Lock is not commonly implemented. The advantage of this for the end user is the phone is now portable from carrier to carrier.  If you move from Vancouver, BC to Milan, Italy with an unlocked device you can subscribe to TiM and the device will work.  If you do not have the device unlocked you will have to continue to use your Rogers SIM and pay roaming charges until you get a new device.

All too frequently, especially with AT&T phones, they are listed on eBay as unlocked when they are not. Always ask the seller what they think unlocked means before assuming it means you can drop a SunCom SIM into an AT&T Z310a and find true happiness. Of the last dozen Z310a phones I purchased on eBay that were listed as unlocked exactly two were. The other ten were not. That is why I usually will include an image of the SmartChip Lock Screen in my auctions and/or images of AT&T and T-Mobile as the service providers on the standby screens. Those images eliminate the ambiguity about what it means to me when I say the phone is unlocked. The first image show the network lock closed, meaning this phone was to be used only on AT&T. The second image shows all locks open. Yes, the first image was from a phone that was listed as unlocked.