>VoiceXML is how the big boys build telephony applications. There are a number of commercial and open source VoiceXML products, and tons of hosting companies. A search for "VoiceXML Hosting" will turn up several thousand options. Many of which will work internationally.
VoiceXML is great for IVR type applications (along with CCXML) but it is not suited to a lot of the types of applications developers use for Twilio/Tropo.
>And of course Tropo, as already mentioned, has an open source core as well as a ton of other fun open source stuff
This statement is true if you guys open source PRISM, your SIP server, but until that happens (has it happened?), it's a little disingenuous to say that Tropo's "core" is open source when it relies on a proprietary SIP server to do all the heavy lifting.