TAPI3 is object oriented. TAPI2 is not. You can do everything with TAPI2, don't worry about IP communication because it is irrelevant for IP office - You can control any extension on the PBX (IP or not). I remember reading some place that TAPI3 lacks an important function (I think it is SetDeviceSpecific or somthing of the sort), so use TAPI2 if you intend to use CTI Link Pro (Lite doesn't support it anyway).