Freeswitch rocks. Great stuff, from a great team too.

If you like Freeswitch, you'd probably like Tropo. Tropo lets you program directly in Javscript and other languages, just like Freeswitch.

Tropo also lets you roll your own. You can download Prism, which include the Tropo engine, here:

http://www.voxeo.com/prism/

I've been in the telephony for 15 years now. If I can help with your explorations at all, feel free to email me: jtaylor at voxeo dot com