What is a SIM-only deal? A SIM card is the small chip that you insert into your phone to tell it who you are and what mobile phone plan you are on. It stands for Subscriber Identity Module and is used to authenticate the user’s information, including a special number that identifies the phone on the network. As the name suggests, a SIM-only plan covers the cost of the SIM – and therefore the cost of being on the network – but not the phone it slots into. That means you’ll need to provide your own handset, whether that’s a case of buying it outright or using a previously owned smartphone.