The invention provides an adapter card for use with a computing apparatus executing in a multitasking environment program elements that implement call-related functional units requiring DSP (Digital Signal Processor) resources. In a specific example, the call-related functional units provide voice recognition services, IP (Internet Protocol) telephony and voice message, among others. The adapter card includes a switch that receives voice signals from one or more telephone lines physically connected to the adapter card. A digital signal processor is provided on the adapter card and can receive an audio signal from the switch for processing. When anyone of the call-related functional units needs access to the resources of the DSP, it issues a control signal to the switch that responds by directing the audio signal that is to be processed by the call-related functional unit to the digital signal processor. The results of the manipulations performed by the digital signal processor on the audio signal are then transferred to the call-related functional unit for further processing. The invention also provides a method and apparatus for processing calls implementing the above-functionality, and a digital signal processor featuring a switch capability coupled to a time-shared DSP resource.