Callsigns is a Tropo-powered, speech-based callsign lookup application!
Callsigns uses tropo.com to handle phone calls and callook.info as the API to lookup data. Check out the recent blog post about it at blog.tropo.com/2011/04/06/tropo-powered-hamradio-callsign-lookup-app/. It uses the Tropo Scripting API and runs entirely in the Tropo cloud using the following communication channels:
Voice and SMS: 1-480-374-3234
VoIP / SIP: sip:firstname.lastname@example.org