The only way to find the root of the cause is to start fop2_server in debug mode with level 15, capturing output. Then stop capturing when the problem occurs, and knowing that someone tried to spy on extension XXX and got YYY instead. If FOP2 is sending the right commands to Asterisk, then we have to inspect the asterisk full log carefully, as it is an Asterisk issue. If FOP2 is sending the wrong command, then it is a FOP2 issue that should be addressed.
Best regards,