I'm attempting to use the 3CX audio conference facility. When I have more than one external member (dialling in from outside the system), those subsequent members are greeted with an engaged tone. The first member can dial in fine. Any number of internal extensions can dial in fine with no issues. I'm nowhere near exceeding "Number of calls in use". I can see my external SIP trunk provider is sending the calls to the 3CX system with no issues, it's simply that the 3CX system will not allow more than one external member on audio conferencing. Any ideas?