Twilio provides a programmatic approach to phone calls much in the same way that we apply business logic through HTTP or XML for complex applications.
OpenVBX is a Web-based phone system that provides for virtual phone numbers as well as a wide range of other functions that are programmable through the OpenVBX plug-in API in PHP OpenVBX lets you take advantage of all of Twilio's integrated services, like text to speech, voice transcription, voice mail forwarding, and SMS messaging, and is integrated with Twilio's back end to handle the connections with carriers as well as for billing and other back-office functions.
Services that combine on-premise and cloud-based services should have a significant advantage over time. Just as IT managers have to get comfortable with consuming cloud services instead of running their own servers, so too do telecommunications managers.
Generally speaking, on-premise PBXs from the likes of Nortel and Cisco are complicated to program and require a fair amount of professional services to configure and maintain. And while voice over IP has reduced the costs of calls, the management of a PBX remains a sunk cost that only large companies keep in-house.
There are quite a few open-source PBX products, with Asterisk the notable leader, as well as a large number of hosted PBX services such as Fonality and VirtualPBX in addition to those offered by big carriers like Qwest. There are also suites of products such as those Voxeo offers for integrated application programming interfaces, instant messaging, and interactive voice response services.
I have little doubt that we'll see more and more hybrid approaches that allow applications to be run as a service or internally. Many would argue that this is the future of a great deal of open-source applications as companies look for more consistent ways to monetize their user base.
Correction 5:02 p.m. PDT: An earlier version of this story incorrectly said that OpenVBX uses Java applets. It does not. OpenVBX provides a range of functions that are programmable through the OpenVBX plug-in API in PHP.