First-Party vs. Third-Party Call Control

While developers of voice-processing servers generally agree that first-party call control can do almost everything third-party call control can do, they prefer the third-party method when it's available. Until recently, because of the high price of CTI (computer/ telephone integration) links, it hasn't been. PC-based voice servers such as those from Active Voice, Applied Voice Technology, and Simpact have traditionally connected to PBXes in the same way that analog phones do.

Asked to connect two parties together, these voice servers do what you would have to do--they dial one party, then they dial the other, and then they make the connection. This procedure ties up costly dial-out ports, and it can take a number of seconds to accomplish. With third-party control such as TSAPI affords, you spare the ports and can make the connection in a fraction of a second.

Superior monitoring of call progress is another advantage of third-party call control. Telephony servers operating from a first-party perspective analyze the status of calls indirectly, by listening for DTMF tones, and they lose control of the calls that they transfer. Servers that have a CTI link can know everything that the switch knows about the status of a call, and they can stay in the loop after connecting two parties, awaiting further instructions.

This conventional wisdom may be changing. The messaging protocols spoken between digital phones and PBXes can be fairly rich. With Voice Technologies Group's digital phone emulator, a telephony server running TAPI (perhaps a Windows NT machine) could effect a fair bit of control over the switch. Installing such a telephony server would be as simple as adding a digital phone--and a lot less scary than putting in a CTI link to the PBX. However, consultant Jim Burton, president of C-T Link (Boston, MA), points out that switchmakers have reason to be wary of opening up to VTG. "If you took the top four phoneset interfaces and put them on a Dialogic board," he says, "you'd have four PBX vendors with good reason to worry about their voice-mail business."

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Illustration: VTG's Scorpion digital phone emulator makes use of the TAPI architecture. It combines a phone, a fax modem, and a sound device on one card. Three key APIs--TAPI, WAV, and the AT modem command set--are supported.