Volume tx rx function

i tried to use function VOLUME to set RX and TX gain on a channel. I tried it on asterisk 1.6 (native) and 1.4 but seems that it never work fine. I have set TX=0 but called and caller can hear perfectly.Have anyone experience using that function?

Below my simple test context in extensions.conf:

exten => 11,1,Set(TX=0)
exten => 11,2,NoOp(“TX:${TX}”)
exten => 11,3,Set(VOLUME(TX)=0)
exten => 11,4,Dial(SIP/[MYPROVIDER]/[MYNUMBER])

Thanks in advance.