Unlocking a mobile phone is a legal process and involves removing the network lock on a phone so it can accept all compatible SIM cards. Unblocking is an illegal activity and isn’t something any trustworthy phone shop or unlocking company will advise on how to do.
Phones are locked to a particular network so the network can subsidise the price of the phone to their customers. This allows the customer to pick up for the phone, often a high priced smartphone, for a cheaper cost than if they were to buy the phone already unlocked, which is also known as “SIM free”.
This is vastly different to a phone that has been blocked. When a phone is blocked, often also known as “barred” or “bricked”, it is because the phone has been reported as either lost or stolen and will no longer work on any network until a verified owner has requested the block be removed from the network. Handling a stolen phone is also an offence in the eyes of the law, and if you come into contact with one, it is advised to contact the police to have the phone recovered and hopefully returned to its original owner.
When a block is put in place, it is put on the phones IMEI (the number to individually identify each phone). This means that when someone inserts a SIM card in the phone and attempts to use for the phone (e.g. for making or receiving phone calls, text messaging or data usage) the phone will not connect to the a networks signal and therefore won’t work. This is because the networks see that the phone's unique IMEI is being attempted for use and it's at this point when the connection is blocked. The phone itself will still load up as usual and all the features, such as MP3 player, calculator, games, etc will still work, but it won’t be able to connect to any network worldwide.
It isn’t rare to see someone unknowingly pick up a blocked phone and attempt to use their SIM card in it. However, when the SIM card doesn’t work, they assume the phone is locked to an alternative network and therefore simply need to unlock the phone to have it work. Unfortunately for them this doesn’t work when the phone is blocked, because even if unlocking it were to be successful (which occurs "locally" on the handset itself and it completely separate to removing a block) the phone will still be blocked and won’t work on any network.
There is a simple check that anyone can do to see if there phone is blocked and that is to insert a SIM card from the phones original network and see if it can receive a signal. Alternatively, there is a paid instant service that we often use that can check the status of the phone. They are a company called CheckMend.
If you are purchasing a handset on a website like eBay and want to check the status of the phone, you can request the IMEI from the seller and use CheckMend to check it’s status before buying.