I launched an almost identical service, http://savethatcall.com, in January. My prices are extremely low (50 cents + 5 cents per minute), but it's hard to compete with free! Good luck.
My margins are low and I've just started gaining traffic, so I haven't made a lot of money. However, I launched the service with a profit motive, so payment integration and realistic pricing were baked in from the start.
I built the site in about 80 hours over 3 or 4 weeks. It was a nice distraction from my master's thesis... Which actually got finished about the same time as the site went live. :)
I used Google App Engine and Twilio, but I also built an audio processing module that runs as a daemon on a Dreamhost server. (The latter was a workaround to GAE's limitations on long-running processes and insertions into the app engine datastore)
How long did it take you to build it? What tech did you use?