Possibly,
You did not mention if 3CX is on-site or hosted. You did not mention if the extension is remote or local. You did not mention the make and model of phone and if there is more than one instance of the extension on the phone. You did not mention if there might be more than one device connected to 3CX that has the same extension. You did not mention what happens to the Q call that came in that reportedly caused the drop.The version of 3CX and the firmware running on the phone would also be nice to know. If memory serves, there was some issue involving Yealink phones and a similar scenario a number of months back, but without details, hard to say.
All of the above may play some part in the scenario.