Mitel compatible softphone

Hi there.Don’t know if anyone is aware of a free softphone that is compatible with the Mitel 3300 ICP PBX?I have about 4 users that needs a softphone on their computers using usb headsets and i cannot seem to find a free softphone solution that is compatible with the Mitel PBX….I have tried Xlite and after configuring it it says ready but does’t actually phone.If i use the “generic SIP phone” device in the Mitel Config is says i need SIP licenses.Isn’t there a way to config it to use 6600 YA PRO device?
Thanks to those with aswers. 🙂