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From their site: BT is undergoing a complete transformation. At the heart of this transformation is our �10 billion 21st century network (21CN) initiative. An IP-based, multi-service network, 21CN is the driver for a radical change. As part of this change, we are kicking off a ground-breaking project, code-named Web21C. The Web21C SDK includes APIs for SMS, Voice Call, Conference Call, Presence, and Location.
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This API is no longer available, as BT has now invested in 'Ribbit' which doesn't seem to have quite the same functionality :'(