also, make sure that you have a correct value retrieved for "External IP" on the default "INFO" tab. If you do not, verify that the machine has the correct DNS information. I had the most luck using DHCP for this.
Did you set the option "PBX delivers Audio" for that extension?
3CX will else hand-off the call to the other party, being an internal phone, a gateway or VOIP provider (i.e. let the two communicate directly instead of through 3CX).