Tropo has the ability to record the audio of calls for you using the record method. To get the recordings, you need an application that will accept an HTTP POST or PUT and then store the audio recordings somewhere for you. Just over a year ago we blogged about using Heroku to host the application and then send those files on to Amazon S3.
If you want the source code to do this your self, it is available on Github here. The original blogpost may be found here. Of course you are not limited to using Heroku and Amazon S3, but this serves as a working example.