I am with LoopyLou on this one, Counterpath's free X-Lite or Bria softphone is the way to go, and Mitel tech support often uses the X-Lite softphone as a standard for testing SIP configuration. The main difference between the two (free/paid version) is the features you can use, the free version is pretty plain jane, the paid has lots of cools stuff (although some of it's features are not supported by the Mitel). Note that to use it effectively you need some kind of edge router that supports SIP, Mitel Border Gateway is the recommended solution here.