Callee hears confirmation sound, should only heard by agent


I just installed a new Grandstream GXW 4008 to connect analog phones to my asterisk server. Unfortunately I am experiencing a new problem. We are playing a confirmation sound when a connection between agent and callee is established (they are put into a conference room via MeetMe). This sound should only be heard by the agent (caller). But the sound is not only heard by the agent but partly by the callee. Someone got an idea? Thanks in advance.

Running Asterisk