can't make 3 way conference calls on grandstream gxp2000

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hi all,
is anyone use this model?
this model should work like this
www.youtube.com/watch?v=hlZ6JqE1MT4

but in my server (TL-MTE, *1.8, PBX-MANAGER 6.1.1.11)
its just doesn't work, it looks like the CONF button is not sending anything to the PBX.