I'm running CCME 4.0 and can set up a 3 way conference for internal numbers but I can establish a conference call with an external number - the phone just displays "Can not create conference"
I'm assuming it's possible but I've just missed a step somewhere - any suggestions?
This sounds like a Transcoding issue. Have a look at this good doc that explains the need for DSP resources for Conferencing External Parties;
Hope this helps!