British Telecom is rumored to be purchasing Web-based telephony provider Ribbit for $55 million, though both companies declined to comment on the possible link-up.

Rumors made their way overnight across the tech and telecom blogospheres (see: TechCrunch; GigaOm; VentureBeat), with the focus on BT acquiring Ribbit to help build a Web-based telephony service that could compete with, among other contenders, Google’s Grand Central offering.

Emails to both BT and Ribbit drew similar replies of declining to comment on rumors.

If true, the pair-up would definitely make sense. BT was one of the first incumbent carriers to deliver a platform-focused SDK and APIs to encourage more open service development with its Web21C project

Back in March, JP Rangaswami, managing director of service design at BT, described the release of CallFlow call management features in Web21C as enabling developers to “build their own Grand Central.”

“The first version of the SDK was about giving someone the ability to build a softphone you could embed within other apps,” Rangaswami said at the time. “With CallFlow, we’re doing the same thing with IVR. Rather than give users a Grand Central-style application, we’re giving developers the tools to build their own.”

In interviews with the SDK development team earlier this spring, they also name-checked Grand Central several times, noting that BT was aiming to build publicly-accessible reference applications similar to Grand Central to help further evangelize the Web21C platform. BT has also discussed building more Web-native, RESTful interfaces for Web21C, also in part to encourage greater development of Web-centric apps.

If they did indeed pick up Ribbit, BT would be getting a more developer-friendly toolkit for building telecom-integrated Web apps. Ribbit, however, is built on top of a Flash-based toolkit rather than on REST and AJAX technologies.

From its start, Ribbit has caught the eye of telecom watchers both for its bold claims – Silicon Valley’s first phone company – as well as its executive ranks that included former telephone industry players. The service includes developer toolkits for building Web/telephony apps and a leased call termination back-end for handling calls and call-backs generated from Ribbit-created services. To date, Ribbit has launched a consumer-grade Web telephony service it calls Amphibian and an application that adds telephony features to Salesforce.com.