In the open source world, you could install Asterisk yourself and use something like Adhearsion to develop your apps. Adhearsion is an open source framework for building Asterisk applications in Ruby. http://adhearsion.com/
Whistle from the 2600hz project could be of interest, too. http://www.2600hz.org/whistle/
To build telephony applications in Java, you can use SIP servlets, a Java standard (look up JSR 289 and JSR 309). Because the Java standard is extremely low-level, we open sourced the framework we use for building Java speech applications. See Moho at http://labs.voxeo.com/moho/ or https://github.com/voxeo/moho
You can also take a look at at VoiceXML, a W3C standard for building telephony apps. 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.
And of course Tropo, as already mentioned, has an open source core as well as a ton of other fun open source stuff. See http://tropo.com/ and https://github.com/tropo/