The move will bring BT up against competitors such as Skype and Google's Android platform. Industry watchers have speculated that BT will bolster its 21CN offering with Ribbit's technology.
Ribbit, which bills itself as 'Silicon Valley's first phone company', offers an 'open platform' for developers, who can create internet-telephony applications and services around this.
JP Rangaswami, managing director of service design at BT, said: "Silicon Valley is emerging as a hotbed of telecommunications innovation.
"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. Buying Ribbit lets us accelerate that vision.”
Ted Griggs, chief executive of Ribbit, said: "The communications industry is entering a new phase. Closed networks are becoming open platforms and developers are now driving innovation.
"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. We are delighted – BT is exactly the partner we were seeking."
The BT deal was on a cash-free, debt free basis. Ribbit will maintain its management team and identity.