For testing purposes, one of the free SIP providers may work just fine. The only thing in addition to that that you will need would be a softphone. In my experience, X-Lite is pretty simple and free. http://www.counterpath.net/x-lite.html
As for the trunk, I would give this a shot:http://www.ipcomms.net/product-freedid.html
That should at least give you enough to test the modem pretty well. Inbound calls are free, outbound are $.02/min.