The official Cisco WebEx Hybrid Audio SDK by Cisco provides VoIP integration for telephony service provider partners. It allows to integrate VoIP client with meeting center to use a method of 'call using computer' to join audio conference calls. To download the zip file, Cisco members should log into an existing account or create a new account.
Twilio has really upped the ante with the release of its new iOS SDK for developing voice over IP applications. This means that any iOS App can quite easily add telephony via the Twilio API to its product. By providing an SDK for iOS, Twilio (a ProgrammableWeb sponsor) has lowered the bar for developers wanting to build VOIP applications. It only makes sense for a company that is a pure-play in the API business to do everything possible to encourage adoption among developers, and this release is another is a series of smart moves by Twilo. I expect that this will have a big impact developer adoption and would love to check back in with Twilio to hear how the project goes over the next couple months.