After Interviewstreet, Morpheus backed Plivo has made it to YCombinator. 
Plivo provides basic building block APIs in the form of Plivo XML and HTTP API, to create telephony apps, so developers can completely do away with learning the nitty-gritty of the telephony plumbing.
Plivo APIs provide functions like Outbound Calling, Fetching Number List, Checking Account Details etc and the company has launched its API platform for voice and SMS applications. Plivo APIs are based on HTTP methods, which make it easy to write applications. You can use any HTTP client in any programming language to interact with the API or even your browser.
Plivo uses FreeSWITCH and runs on its own dedicated servers. Plivo was part of Morpheus batch and interestingly, the founders met over GitHub , while trying to integrate telephony into their web applications. The service is currently available only in US and Canada, though one can use their own outgoing carriers or phone numbers (i.e. you can buy numbers from your carrier and integrate with Plivo).