To hook up a regular (PSTN) phone line to your local computer, you must have a voice modem. To send and receive faxes in addition to making voice or data calls, you must have a fax modem.
To use IP telephony service providers (available from third-party vendors), your computer must have a sound card, video camera, speakers, and a microphone (or headset).
To support speech and hearing simultaneously, you must have a full-duplex sound card.
To include video communications, you must have a video capture card.
You might need to install other hardware for special telephony programs. For example, to connect trunk lines for a personal computer-based telephony server or cables to connect to a PBX system, you might need expansion cards. For additional information about your telephony hardware requirements, consult your hardware vendor or hardware documentation.