Service providers are not required to support phone devices. Line and phone devices are independent devices. Service providers that fully implement this independence can offer uses of the line and phone devices to the applications that are not defined by normal telephony protocols. For example, you can use the handset of the desktop's phone as a wave form audio device for voice recording or playback, and, depending on the implementation, you may be able to do it without the switch knowing that the phone is in use. In such an implementation, lifting the local phone handset may not automatically send an offhook signal to the switch. This feature also allows your application to ring the local phone, independently of incoming calls. Capabilities of actual service providers may be limited by the hardware and software used to interconnect the switch, the phone, and the computer. TSPI includes functions to retrieve device capabilities that allow clients to determine whether such a usage model is supported.