The move will give the communications giant access to a burgeoning group of developers, as well as put it in competition with other "telco 2.0" services like Skype and Google's GrandCentral.
"Silicon Valley is emerging as a hotbed of telecommunications innovation," said J.P Rangaswami, managing director of service design at BT, in a statement. "With Ribbit, not only do we extend our presence in the Valley, but we also gain a groundbreaking platform, a growing community of developers, and a world-class team that share a common vision."
Ribbit, which dubs itself "Silicon Valley's first phone company," created a platform that allows developers to integrate voice communications into software and Web sites. The company's software-based call-switching technology allows developers to tie voice from any Flash-enabled browser to a public telephone network or VoIP networks, including calls from services like Google Talk and Skype.
Ribbit's platform has attracted thousands of developers since its introduction, according to the company. For example, Salesforce.com used the technology to let sales and marketing managers integrate cellular calls with Salesforce applications.
"The communication industry is entering a new phase. Closed networks are becoming open platforms and developers are now driving innovations," said Ted Griggs, Ribbit's chief executive, in a statement. "By adding Ribbit's capability to the power of BT's global 21CN platform, we will now be able to give the development community the tools they need to innovate on a global scale."
Ribbit, which was founded in 2006, will retain its name and management team.