A SIM-only plan is a mobile phone service that provides only the SIM card without a new phone. This type of plan allows users to keep their existing devices while enjoying a mobile network's services, such as calls, texts, and data. It is often more affordable than traditional contracts that include a new phone. These plans typically come with flexible contract lengths, ranging from month-to-month to longer commitments. Users can choose from various data allowances and features, making it easier to find a plan that suits their needs. Popular providers of SIM-only plans include Verizon, AT&T, and T-Mobile.