With VoIP networks making headway in terms of performance and reliability, the idea of a softphone -- an application loaded onto a computing device that establishes a voice path to another party -- replacing the desk phone is more plausible now than when we first tested these wares five years ago.

Softphones allow for all of the basic telephony functions provided by traditional phones, such as hold, transfer and call waiting. Once the application is launched and a connection established with a VoIP-based back end, users can click on a graphical number pad (see slide show of softphone interfaces) and use the microphone and speakers built into their computer to make a call. For better sound quality and performance, you can use a headset, such as one from Plantronics, which plugs into a USB port and has built-in digital signal processors that yield clarity that rival traditional handsets.

Many softphone applications take advantage of being on the computer and enhance the work environment with integrated connections to popular contact-management packages, and click-to-dial and contact screen pop-ups for incoming calls. Softphones can appear as an exact graphical representation of the desk phone or provide a thin toolbarlike interface to minimize the amount of space they take up on the user's desktop.

Support for Session Initiation Protocol (SIP) has grown tremendously and has opened up avenues for broader device support among vendors. That includes third-party softphones, which can interact with any back-end phone system that supports SIP endpoints (see test of third-party SIP softphones).

SIP-based softphone projects also are flourishing in the open source realm (see test of open source softphone options). Nearly all IP PBX vendors provide a softphone for use with their systems. Vendors sell and license these like their hard phones (see story about proprietary softphones).

The drawback to softphones lies in using the Internet for voice service when users are traveling. Some of the vendor-specific softphones address this issue by letting users specify an alternative phone number to establish the voice path, such as a cell phone or analog line, while using the softphone to communicate with the PBX and supply the call handling features.