Definitely biased here, since I work for Voxeo Labs, but Tropo is free for development, supports SIP, SMS, Shortcodes, TFN's ITFN's, ect. We also have great support ( 24/7 ) and fantastic documentation. Our product is also available on premise, should you want to go that route. It is also being deployed into many large carrier clouds, such as AT&T, Deutsch Telekom, ect.