There are many different types of caller ID devices available, and they are generally classified based on which services they offer and how they offer those services. Some, for instance, only tell the phone number of the person calling, while others identify the caller's name and location. Additionally, some devices are separately attached to a phone or phone line while others are fully integrated with the existing phone service. Different ones work on analog, digital, and Internet-based phone services, though most modern caller ID systems are not separate devices. Some phone service providers require customers to pay extra for caller identification, but others include it as a basic service.
Some caller ID devices, particularly older ones, are separate from the phone itself. The separate devices are generally responsible for decoding the incoming call in order to determine the number of the phone at the call's origin. In many cases, such devices also display the number, though others simply transmit the information to the phone, allowing it to display the phone number.
These devices may also be integrated with a call waiting service that allows someone talking on the phone to identify a second caller and to decide whether to switch calls while leaving both callers on the phone, to terminate one call in favor of the other, or to choose one of a variety of other options. Most modern phone services include some form of call waiting as a built-in feature.
Different types of caller ID devices are optimized for different types of devices. Some voice over Internet protocol (VoIP) programs use software-based caller ID and do not require dedicated devices at all. Many phones, particularly cellular phones, have built-in caller ID software or use an external service from the phone company. Many offer caller ID without any additional cost, and some do not even allow the option to turn it off. Others offer a basic caller ID service but charge for additional functionality, such as identifying the geographic origin of a call.
In some cases, these devices can be used with a variety of different household electronics. An incoming telephone call, for instance, may register with an individual's telephone, television, and computer. The information can be displayed on all of these devices, and in many cases, the individual can choose to answer the phone or terminate the call without ever reaching for the phone itself.